French Onion Puff Pastry Tart (Printable Version)

Flaky puff pastry layered with caramelized onions, melted Gruyère and thyme for a golden, savory tart.

# What You'll Need:

→ For the Tart

01 - 1 sheet puff pastry (about 8.8 oz), thawed if frozen
02 - 3 large yellow onions, peeled and thinly sliced
03 - 2 tablespoons unsalted butter
04 - 1 tablespoon olive oil
05 - 1 teaspoon granulated sugar
06 - 1/2 teaspoon fine salt
07 - 1/4 teaspoon freshly ground black pepper
08 - 1 teaspoon fresh thyme leaves, plus extra for garnish
09 - 1 cup (about 4.2 oz) Gruyère, grated
10 - 1 large egg, beaten (for egg wash)

→ Garnish (optional)

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

# Steps:

01 - Preheat the oven to 400°F. Line a baking sheet with parchment paper and set aside.
02 - Heat a large skillet over medium heat. Melt the butter with the olive oil, add the sliced onions, sugar, salt and pepper, and cook, stirring frequently, for 20–25 minutes until the onions are deeply golden and soft. Stir in the thyme leaves during the final 5 minutes. Remove from heat and allow to cool slightly.
03 - On a lightly floured surface, roll the sheet of puff pastry to roughly a 10 x 14-inch rectangle. Transfer it to the prepared baking sheet. Using a sharp knife, score a 1/2-inch border around the edge without cutting through the dough and dock the inner area all over with a fork to prevent excessive puffing.
04 - Brush the scored border with the beaten egg. Spread the caramelized onions evenly within the border, then scatter the grated Gruyère over the onions in an even layer.
05 - Bake on the middle rack for 18–22 minutes, or until the pastry is fully puffed and a deep golden brown.
06 - Remove from the oven and let rest for 5 minutes. Garnish with additional thyme leaves and a grind of black pepper. Slice and serve warm or at room temperature.

# Expert Advice:

01 -
  • The secret is in caramelizing the onions until they're almost jammy—the flavor in every bite is deep and savory beyond expectation.
  • Even my friend who claims to avoid onions came back for seconds; it’s astonishing how the Gruyère melts into the pastry and makes each bite irresistible.
02 -
  • If you rush the onions, the tart will lack the sweet depth that makes it unforgettable—patience pays off here.
  • Letting the pastry sit too long on the counter can make it sticky and harder to handle; keep it cool for best layering.
03 -
  • Use a serrated knife for the cleanest slices—straight blades can crush the delicate layers.
  • Brushing the pastry border with extra egg wash halfway through baking makes it extra shiny and golden, if you’re aiming for pastry perfection.
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