# Components:
→ 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 extra-virgin 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 (about 4 oz)
10 - 1 large egg, beaten (for egg wash)
→ Garnish (optional)
11 - Additional fresh thyme sprigs
12 - Freshly ground black pepper
# Directions:
01 - Preheat the oven to 400°F. Line a baking sheet with parchment paper and set aside.
02 - Heat the butter and olive oil in a large skillet over medium heat. Add the sliced onions, sugar, salt, and pepper. 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, then remove the pan from the heat and allow the onions to cool slightly.
03 - On a lightly floured surface, roll the puff pastry to a rectangle approximately 10 x 14 inches. Transfer the sheet to the prepared baking sheet, aligning the edges so the pastry lies flat.
04 - Using a sharp knife, score a 1/2-inch border around the perimeter of the pastry without cutting all the way through. Prick the interior area all over with a fork to prevent excessive rising in the center.
05 - Brush the scored border evenly with the beaten egg to promote a glossy, golden finish.
06 - Spread the caramelized onions evenly within the scored border, then sprinkle the grated Gruyère over the onions in an even layer.
07 - Bake in the preheated oven for 18–22 minutes, or until the pastry is well puffed and deep golden brown at the edges.
08 - Remove from the oven and allow to rest for 5 minutes. Garnish with extra thyme and a crack of black pepper if desired, slice into portions and serve warm or at room temperature.
