French Onion Puff Pastry Tart (Printable)

Flaky puff pastry crowned with caramelized onions, nutty Gruyère and fresh thyme for a warm, savory tart.

# What You'll Need:

→ For the Tart

01 - 1 sheet puff pastry (about 9 ounces), thawed
02 - 3 large yellow onions, 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 grated Gruyère cheese (about 4.2 ounces)
10 - 1 large egg, beaten (for egg wash)

→ Garnish (optional)

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

# How to make it:

01 - Preheat the oven to 400°F and line a baking sheet with parchment paper; set aside.
02 - Heat the butter and olive oil in a large skillet over medium heat, add the sliced onions, sugar, salt and pepper, and cook, stirring frequently, until deeply golden and caramelized, about 20–25 minutes; stir in the thyme during the final five minutes, then remove from heat and cool slightly.
03 - On a lightly floured surface, roll the puff pastry to approximately a 10 x 14 inch rectangle and transfer it to the prepared baking sheet.
04 - With a sharp knife, score a 1/2-inch border around the edge of the pastry without cutting through, then prick the interior all over with a fork to prevent excessive puffing.
05 - Brush the scored border lightly with the beaten egg to promote a golden finish.
06 - Spread the caramelized onions evenly inside the scored border and sprinkle the grated Gruyère over the onions in an even layer.
07 - Bake on the middle rack for 18–22 minutes, or until the pastry is fully puffed and deep golden brown.
08 - Remove from the oven and let rest for 5 minutes, garnish with additional thyme and a grind of black pepper, then slice and serve warm or at room temperature.

# Recipe Expert Tips:

01 -
  • Nobody expects how wildly flavorful such a simple tart can be—the secret is all in those beautifully caramelized onions.
  • This tart’s flaky crust and cheesy top make it just as marvelous with a glass of wine as it is as a brunch centerpiece.
02 -
  • Once I rushed and turned up the heat, thinking faster meant better—my onions burned in minutes; slow and steady wins here.
  • I never realized until trying that pricking the center of the pastry really prevents a soggy, domed tart—don’t skip that step.
03 -
  • When caramelizing onions, use a wide skillet so they aren’t crowded—they’ll brown evenly.
  • A sprinkle of fresh thyme just before serving lifts each bite and makes the tart feel bakery-fresh.
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