French Onion Puff Pastry Tart (Printer-friendly)

Thin, crispy puff pastry topped with caramelized onions, Gruyère and thyme — serve warm or at room temperature.

# What You'll Need:

→ Tart

01 - 1 sheet puff pastry (about 9 oz), thawed if frozen
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
09 - 1 cup grated Gruyère (about 4.25 oz)
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 oven to 400°F. Line a baking sheet with parchment paper and set aside.
02 - Heat a large skillet over medium heat, add butter and olive oil. Add the sliced onions, sugar, salt and pepper and cook, stirring frequently, until deeply golden and soft, about 20–25 minutes; stir in the thyme during the final 5 minutes. Remove from heat and let cool slightly.
03 - On a lightly floured surface, roll the puff pastry to a rectangle approximately 10 x 14 inches. Transfer the pastry to the prepared baking sheet.
04 - Using a sharp knife, score a 1/2-inch border around the edge without cutting through. Prick the inner area all over with a fork to prevent excessive rising.
05 - Brush the scored border with the beaten egg to promote an even, golden crust.
06 - Spread the caramelized onions evenly inside the scored border, then scatter the grated Gruyère over the top.
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 crack of black pepper, slice and serve warm or at room temperature.

# Expert Suggestions:

01 -
  • This tart tricks everyone into thinking you spent all day in the kitchen—the flavor is pure bistro magic with minimal fuss.
  • It's the perfect solution for using up onions and impressing friends with very little effort.
02 -
  • The time spent on caramelizing onions is sacred—rushing this step leaves them pale and bland instead of jammy and sweet.
  • I used to skip the scoring and docking, only to find my tart ballooning in mysterious shapes—now, it's always picture perfect.
03 -
  • Let the onions cool just briefly before assembling—adding them piping hot can make the pastry soggy.
  • A generous brushing of egg wash on the border gives you the bakery-style golden sheen that impresses every time.
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