French Onion Puff Pastry Tart (Print Version)

Flaky pastry layered with sweet caramelized onions, melted Gruyère and fragrant thyme.

# What You'll Need:

→ For the Tart

01 - 1 sheet (about 250 g) puff pastry, thawed if frozen
02 - 3 large yellow onions, thinly sliced
03 - 2 tbsp unsalted butter
04 - 1 tbsp olive oil
05 - 1 tsp sugar
06 - 1/2 tsp salt
07 - 1/4 tsp freshly ground black pepper
08 - 1 tsp fresh thyme leaves (plus extra for garnish)
09 - 120 g Gruyère cheese, grated
10 - 1 egg, beaten (for egg wash)

→ Garnish (optional)

11 - Extra fresh thyme sprigs
12 - Freshly ground black pepper

# How to Prepare:

01 - Preheat the oven to 400°F (200°C). Line a baking sheet with parchment paper.
02 - In a large skillet over medium heat, melt the butter with the olive oil. Add the sliced onions, sugar, salt, and pepper. Cook, stirring often, for 20–25 minutes until the onions are deeply golden and caramelized. Stir in the thyme leaves during the last 5 minutes of cooking. Remove from heat and let cool slightly.
03 - Roll out the puff pastry on a lightly floured surface to a rectangle about 10 x 14 inches (25 x 35 cm). Transfer to the prepared baking sheet.
04 - Using a sharp knife, score a 1/2-inch (1.5 cm) border around the edge of the pastry, being careful not to cut all the way through. Prick the inside area all over with a fork.
05 - Brush the border with beaten egg.
06 - Evenly spread the caramelized onions inside the border. Sprinkle the Gruyère cheese over the onions.
07 - Bake for 18–22 minutes or until the pastry is puffed and golden brown.
08 - Remove from the oven and let cool for 5 minutes. Garnish with extra thyme and black pepper if desired. Slice and serve warm or at room temperature.

# Expert Advice:

01 -
  • This tart tastes like the very best part of French onion soup, all buttery and rich but so much easier to serve.
  • Every bite is both crisp and melty, making it irresistible for gatherings or a cozy night for two.
02 -
  • Once, I tried to rush the onions on too-high heat and ended up with burnt bits instead of sweet ribbons—lower and slower is non-negotiable.
  • The difference between store-brand and all-butter puff pastry shocked me: flakiness and depth of flavor are worlds apart.
03 -
  • If your onions start to stick, splash in a tablespoon of water or even dry white wine to pick up all the flavor from the pan.
  • For extra golden color, rotate the baking sheet halfway through baking and keep an eye on your oven—mine runs a little hot on the left.
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