Name: Golden Caramelized Onion & Gruyère Puff Pastry Tart Recipe
Prep time: 15 min
Cook time: 40 min
Total time: 55 min
Cuisine: French
Ingredients: 1 sheet puff pastry, thawed (about 10×10 inches); 1 egg, beaten (for brushing edges); 1 tbsp Dijon mustard (optional, for flavor depth); 1 cup (100 g) shredded Gruyère cheese; ¼ cup (60 ml) heavy cream; 1 egg; ½ tsp black pepper; 1 tsp fresh thyme leaves (plus more for garnish); Salt, to taste; Sea salt, for finishing; 3 large yellow onions, thinly sliced; 2 tbsp butter; 1 tbsp olive oil; 1 tsp sugar

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Introduction

This Golden Caramelized Onion & Gruyère Puff Pastry Tart is a perfect blend of sweet, savory, and cheesy flavors on a flaky pastry base. It makes a wonderful appetizer or light meal that’s both elegant and easy to prepare.

A rectangular flat pastry dough laid on a baking sheet lined with parchment paper rests on a wooden board. The dough is topped with three visible layers: a base layer of cooked onions with a golden brown and slightly translucent texture, a middle layer of dark brown mushrooms scattered unevenly, and a top layer of shredded pale yellow cheese generously spread over the entire surface. The background shows a white marbled kitchen countertop with various blurred kitchen items, including a patterned ceramic container. A woman's hand is reaching over the pastry from the left side. Photo taken with an iphone --ar 4:5 --v 7

Ingredients

1 sheet puff pastry, thawed (about 10×10 inches)
1 egg, beaten (for brushing)
1 tbsp Dijon mustard (optional, for flavor depth)
3 large yellow onions, thinly sliced
2 tbsp butter
1 tbsp olive oil
1 tsp sugar
Salt, to taste
1 tsp fresh thyme leaves (plus more for garnish)
1 cup (100 g) shredded Gruyère cheese
¼ cup (60 ml) heavy cream
1 egg
½ tsp black pepper

Instructions

Step 1: In a large skillet, melt butter with olive oil over medium heat.
Step 2: Add sliced onions, sugar, and a pinch of salt.
Step 3: Cook slowly, stirring often, for 25–30 minutes, until onions are golden brown and jammy.
Step 4: Stir in thyme and set aside to cool slightly.
Step 5: Preheat oven to 400°F (200°C).
Step 6: Place puff pastry on a parchment-lined baking sheet.
Step 7: Score a 1-inch border around the edges (don’t cut through).
Step 8: Lightly prick the inner section with a fork to prevent puffing too much.
Step 9: Brush edges with beaten egg and bake for 10 minutes, until lightly golden.
Step 10: In a bowl, whisk together cream, egg, pepper, and Gruyère.
Step 11: Spread Dijon mustard (if using) over the pre-baked pastry base.
Step 12: Pour the cheese mixture evenly within the scored border.
Step 13: Top generously with caramelized onions.
Step 14: Return to oven and bake for 15–18 minutes, until the tart is puffed and golden brown.
Step 15: Sprinkle with fresh thyme and a touch of sea salt.
Step 16: Slice into squares and serve warm or at room temperature.

Tips & Variations

Use a combination of Gruyère and sharp white cheddar for a stronger cheese flavor.
Adding a splash of balsamic vinegar to the onions while caramelizing enhances their sweetness.
For extra crispiness, chill the baked pastry before adding the toppings.
Substitute fresh thyme with rosemary or sage for a different herb aroma.

Storage

Store leftover tart in an airtight container in the refrigerator for up to 3 days. Reheat gently in a low oven (around 300°F/150°C) to restore crispness, or warm in a skillet over low heat. This tart is best enjoyed fresh but can also be served cold or at room temperature.

How to Serve

A round tart with a golden, flaky crust forms the base layer, topped with a thick, creamy white cheese filling that appears smooth and rich. On top, there are evenly spaced caramelized onion rose shapes with a deep brown color and slightly shiny texture, adding a decorative and appetizing look. Small green herb leaves are scattered over the onions, giving a fresh contrast to the warm tones. The tart is presented on a wooden cutting board with some fresh herb sprigs nearby, and the background is a white marbled texture. Photo taken with an iphone --ar 4:5 --v 7

Serve this delicious recipe with your favorite sides.

FAQs

Can I use frozen puff pastry instead of fresh?

Yes, frozen puff pastry works perfectly. Just make sure it is fully thawed before using, so it’s easier to handle and bakes evenly.

Is it possible to make this tart ahead of time?

You can prepare the caramelized onions in advance and keep them refrigerated for up to 3 days. Assemble and bake the tart just before serving for the best texture and flavor.