Slow-cooked onions transform into a sweet, jammy foundation for this Caramelized Onion and Gruyere Tart. It’s a rustic yet elegant dish perfect for dinner or as an impressive appetizer. The combination of rich onions, nutty Gruyère, and buttery puff pastry is pure perfection.
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Why You’ll Love This Caramelized Onion and Gruyere Tart
- Deceptively simple: Looks fancy but the process is straightforward.
- Huge flavor payoff: Sweet, umami onions pair beautifully with nutty Gruyère.
- Incredibly versatile: Serve warm, as an appetizer, or at room temperature.
- Dreamy texture: Flaky crust, melting onions, and gooey cheese in every bite.
Ingredients & Tools
- 1 sheet of store-bought puff pastry, thawed
- 3 large yellow onions (about 1.5 lbs / 700 g), thinly sliced
- 2 tbsp unsalted butter
- 1 tbsp olive oil
- 1 tsp fresh thyme leaves, plus extra for garnish
- 1 tsp balsamic vinegar
- 1/4 tsp granulated sugar (optional, to help caramelization)
- 1 cup (about 100 g) Gruyère cheese, freshly grated
- 1/4 cup (about 25 g) grated Parmesan cheese
- 1 large egg, for egg wash
- Salt and freshly ground black pepper to taste
Tools: A large skillet, a rolling pin, a baking sheet, and parchment paper.
Notes: Don’t rush the onions—low and slow cooking builds foundational flavor. Freshly grated Gruyère melts better than pre-shredded.
Nutrition (per serving)
| Calories: | 420 kcal |
| Protein: | 12 g |
| Fat: | 28 g |
| Carbs: | 30 g |
| Fiber: | 2 g |
Serves: 6 | Prep Time: 20 minutes | Cook Time: 50 minutes | Total Time: 1 hour 10 minutes
Before You Start: Tips & Ingredient Notes
- Patience is your secret ingredient. Caramelizing onions properly cannot be rushed over high heat. It will take a good 30-40 minutes. If you try to speed it up, you’ll just burn them and end up with a bitter taste instead of a sweet, deep flavor.
- Why a mix of butter and oil? The butter gives the onions incredible flavor, while the olive oil raises the smoke point, preventing the butter from burning during the longer cooking time. It’s a little trick that gives you the best of both worlds.
- Don’t skip the balsamic. A tiny splash of balsamic vinegar at the end of cooking the onions isn’t really for acidity—it’s a flavor enhancer. It deepens the color and adds a subtle, complex sweetness that makes the onions even more incredible.
- Thaw your pastry properly. Take the puff pastry out of the freezer and let it thaw slowly in the fridge for a few hours, or on the counter for about 40 minutes. If it’s too cold, it will crack; if it’s too warm, it will be sticky and hard to handle.
How to Make Caramelized Onion and Gruyere Tart
Step 1: Start by caramelizing your onions. This is the heart of the dish, so give it the time it deserves. In a large skillet over medium-low heat, melt the butter with the olive oil. Add all of your thinly sliced onions and give them a good stir to coat them in the fat. Sprinkle with a pinch of salt and the optional sugar. You’ll notice the onions will start to sizzle gently—that’s what you want. Now, the waiting game begins.
Step 2: Cook the onions slowly, stirring every 5-7 minutes. You’re not looking for a quick sauté here. After about 10 minutes, they’ll become soft and translucent. After 20 minutes, they’ll start to take on a pale golden color. This is where the magic happens. Keep stirring occasionally, scraping any browned bits from the bottom of the pan. If the pan seems dry or the onions are sticking, you can add a tablespoon of water to deglaze the pan and keep things moving.
Step 3: After about 30-35 minutes, the onions should be a deep, golden-brown color, soft, and jammy. They will have reduced in volume significantly. Now, stir in the fresh thyme leaves and the balsamic vinegar. Cook for one more minute, then remove the skillet from the heat. Taste and season generously with black pepper and more salt if needed. Let this mixture cool slightly while you prepare the pastry.
Step 4: Preheat your oven to 400°F (200°C). On a lightly floured surface, roll out your thawed puff pastry sheet just enough to smooth out any creases. Transfer it to a baking sheet lined with parchment paper. Using a sharp knife, lightly score a border about 1 inch (2.5 cm) from the edge all the way around—be careful not to cut all the way through. This border will puff up dramatically and create a beautiful, rustic frame for your tart.
Step 5: Now for the assembly! Evenly sprinkle about two-thirds of the grated Gruyère and all of the Parmesan cheese over the inner rectangle of the pastry (inside the scored border). This cheesy base layer creates a barrier that helps prevent the pastry from getting soggy from the onion mixture. Then, spoon your glorious caramelized onions over the cheese, spreading them into an even layer.
Step 6: Top the onions with the remaining Gruyère cheese. In a small bowl, beat the egg with a teaspoon of water to make an egg wash. Use a pastry brush to carefully brush the egg wash over the raised border of the pastry. This is what will give it that gorgeous, deep golden-brown and shiny finish.
Step 7: It’s baking time! Place the tart in the preheated oven and bake for 20-25 minutes. You’re looking for the pastry to be puffed and a rich golden brown, and the cheese on top should be melted, bubbly, and spotted with darker golden bits. The whole kitchen will smell absolutely incredible.
Step 8: Once it’s out of the oven, let the tart rest on the baking sheet for at least 5-10 minutes. I know it’s tempting to cut in right away, but this resting time allows the cheesy onion filling to set slightly, making it much easier to slice cleanly. Garnish with a few extra fresh thyme leaves before serving.
Storage & Freshness Guide
- Fridge: Store leftovers in an airtight container for up to 3 days.
- Freezer: Wrap tightly and freeze baked tart for up to 1 month.
- Reviving: Reheat in a 350°F (175°C) oven for 10 minutes to crisp pastry.
Serving Suggestions
Complementary Dishes
- A simple arugula salad — The peppery bite of arugula dressed with a sharp lemon vinaigrette cuts through the richness of the tart beautifully, creating a perfect balanced plate.
- Roasted tomato soup — For the ultimate comfort meal, serve a warm wedge of this tart alongside a bowl of creamy, slightly sweet roasted tomato soup. It’s a classic pairing for a reason.
- Sautéed garlic mushrooms — If you want to double down on the earthy, umami flavors, a side of savory mushrooms cooked with plenty of garlic is an absolute winner.
Drinks
- A dry white wine like Sauvignon Blanc — Its crisp acidity and citrus notes provide a refreshing contrast to the savory, cheesy depth of the tart, cleansing the palate between bites.
- A light-bodied red like Pinot Noir — For red wine lovers, a fruit-forward Pinot Noir has enough acidity to stand up to the dish without overpowering the delicate flavors of the onion and Gruyère.
- Sparkling apple cider — A non-alcoholic option that’s just as celebratory. The bubbles and slight sweetness are a fantastic foil for the salty, savory notes of the tart.
Something Sweet
- Dark chocolate pots de crème — The intense, bitter-sweet chocolate is a decadent and simple way to end the meal, offering a complete shift in flavor that feels incredibly satisfying.
- Poached pears with a hint of cinnamon — The gentle, spiced sweetness of the pears feels light and elegant after the savory tart, and it continues that lovely autumnal vibe.
- Lemon sorbet — Nothing cleanses the palate and provides a bright, zingy finish quite like a scoop of intensely lemony sorbet. It’s refreshing and surprisingly perfect.
Top Mistakes to Avoid
- Mistake: Slicing the onions too thickly. Thin, even slices are crucial for uniform caramelization. If some pieces are thick, they’ll remain crunchy and undercooked while others burn.
- Mistake: Cooking the onions over high heat. I’ve messed this up before too… impatience leads to burning. The sugars in the onion need time to break down slowly. Medium-low heat is your friend here.
- Mistake: Forgetting to score the pastry border. If you don’t score that inner border, the entire pastry will puff up evenly and the filling can spill over the edges. The scored line guides the puff and creates that beautiful frame.
- Mistake: Cutting the tart immediately. Slicing into it straight from the oven will cause the molten cheese and onion filling to ooze out everywhere. A brief rest is non-negotiable for picture-perfect slices.
Expert Tips
- Tip: Caramelize a big batch in advance. Caramelized onions freeze beautifully. Make a double or triple batch on a lazy Sunday, freeze them in portions, and you can whip up this tart on a weeknight in no time.
- Tip: Add a layer of mustard. For an extra flavor dimension, lightly spread a thin layer of Dijon mustard over the pastry base (inside the border) before adding the cheese. It adds a wonderful tangy background note.
- Tip: Use a pizza cutter for clean slices. When it’s time to serve, a pizza cutter or a large, sharp chef’s knife will glide through the flaky pastry and give you the cleanest cuts without squashing the tart.
- Tip: Re-crisp leftovers in the oven. If you have leftovers, reheat them in a toaster oven or a conventional oven at 350°F (175°C) for about 10 minutes. This will bring back the crispness to the pastry, unlike a microwave which will make it soft and soggy.
FAQs
Can I make this tart ahead of time?
Absolutely, you can! You have two great options. First, you can fully assemble the tart (up to the point of baking), cover it tightly, and refrigerate it for up to 24 hours. Just add a few extra minutes to the baking time since it will be going into the oven cold. Alternatively, you can caramelize the onions up to 3 days in advance and store them in the fridge, making assembly on the day super quick.
What can I use instead of Gruyère cheese?
Gruyère is ideal for its excellent melting quality and nutty flavor, but there are good substitutes. Comté is very similar. A good Swiss cheese or even Emmental would work well. For a stronger flavor, a young Fontina is a fantastic choice. I’d avoid very hard or very soft cheeses as they won’t provide the same texture.
My onions are taking forever to caramelize. What’s wrong?
This almost always comes down to heat and crowding. If your pan is too small and the onions are piled high, they’ll steam rather than caramelize. Make sure you’re using a large, wide skillet. Also, double-check that your heat is at a steady medium-low—it should be a gentle sizzle, not a vigorous fry. Patience is key, I promise it’s worth the wait!
Can I use a different type of pastry?
Yes, though the result will be different. Puff pastry gives you that light, flaky, buttery layers. You could use a shortcrust pastry for a more sturdy, cookie-like base, which is also delicious. If using shortcrust, you might want to blind-bake it for 10-15 minutes before adding the filling to ensure the bottom gets crisp.
How should I store and reheat leftovers?
Store any leftover tart in an airtight container in the refrigerator for up to 3 days. For reheating, avoid the microwave if you can—it will make the pastry soft. Instead, reheat slices in a 350°F (175°C) oven or a toaster oven for about 10 minutes, or until warmed through and the pastry is crisp again.
Caramelized Onion And Gruyere Tart
Learn how to make an impressive Caramelized Onion and Gruyere Tart with this easy recipe. Perfect for dinner or as an appetizer. Get the recipe now!
Ingredients
For the tart:
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1 sheet puff pastry (thawed)
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3 large yellow onions (thinly sliced)
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2 tbsp unsalted butter
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1 tbsp olive oil
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1 tsp fresh thyme leaves (plus extra for garnish)
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1 tsp balsamic vinegar
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1/4 tsp granulated sugar
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1 cup Gruyère cheese (freshly grated)
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1/4 cup grated Parmesan cheese
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1 large egg
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Salt and freshly ground black pepper (to taste)
Instructions
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In a large skillet over medium-low heat, melt the butter with the olive oil. Add all of your thinly sliced onions and give them a good stir to coat them in the fat. Sprinkle with a pinch of salt and the optional sugar.01
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Cook the onions slowly, stirring every 5-7 minutes. After about 10 minutes, they’ll become soft and translucent. After 20 minutes, they’ll start to take on a pale golden color. Keep stirring occasionally, scraping any browned bits from the bottom of the pan. If the pan seems dry or the onions are sticking, you can add a tablespoon of water to deglaze the pan and keep things moving.02
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After about 30-35 minutes, the onions should be a deep, golden-brown color, soft, and jammy. Stir in the fresh thyme leaves and the balsamic vinegar. Cook for one more minute, then remove the skillet from the heat. Taste and season generously with black pepper and more salt if needed. Let this mixture cool slightly while you prepare the pastry.03
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Preheat your oven to 400°F (200°C). On a lightly floured surface, roll out your thawed puff pastry sheet just enough to smooth out any creases. Transfer it to a baking sheet lined with parchment paper. Using a sharp knife, lightly score a border about 1 inch (2.5 cm) from the edge all the way around—be careful not to cut all the way through.04
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Evenly sprinkle about two-thirds of the grated Gruyère and all of the Parmesan cheese over the inner rectangle of the pastry (inside the scored border). Then, spoon your glorious caramelized onions over the cheese, spreading them into an even layer.05
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Top the onions with the remaining Gruyère cheese. In a small bowl, beat the egg with a teaspoon of water to make an egg wash. Use a pastry brush to carefully brush the egg wash over the raised border of the pastry.06
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Place the tart in the preheated oven and bake for 20-25 minutes. You’re looking for the pastry to be puffed and a rich golden brown, and the cheese on top should be melted, bubbly, and spotted with darker golden bits.07
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Once it’s out of the oven, let the tart rest on the baking sheet for at least 5-10 minutes. Garnish with a few extra fresh thyme leaves before serving.08
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