# Components:
→ Pastry
01 - 1 sheet frozen puff pastry, thawed (approximately 8.8 ounces)
→ Onions
02 - 3 large yellow onions, thinly sliced
03 - 2 tablespoons unsalted butter
04 - 1 tablespoon olive oil
05 - 1 teaspoon sugar
06 - 1/2 teaspoon salt
07 - 1/4 teaspoon freshly ground black pepper
→ Filling & Topping
08 - 1/2 cup crème fraîche or sour cream (4 fluid ounces)
09 - 1 teaspoon Dijon mustard
10 - 1/2 cup grated Gruyère cheese (2.1 ounces)
11 - 1 teaspoon fresh thyme leaves or 1/2 teaspoon dried thyme
12 - Optional: 1 tablespoon chopped fresh chives for garnish
# Preparation Steps:
01 - Set the oven to 400°F and line a baking sheet with parchment paper.
02 - Melt butter with olive oil in a large skillet over medium heat. Add sliced onions, sugar, salt, and pepper. Cook, stirring often, until onions are deeply golden and caramelized, about 25 to 30 minutes.
03 - Roll out the thawed puff pastry on a lightly floured surface into a 12-inch square or circle. Transfer it to the prepared baking sheet.
04 - Combine crème fraîche and Dijon mustard in a small bowl. Spread the mixture evenly over the pastry, leaving a 1-inch border around the edges.
05 - Sprinkle half the grated Gruyère over the crème mixture. Spread the caramelized onions on top, then sprinkle with thyme and the remaining Gruyère cheese.
06 - Fold the pastry edges gently over the filling to form a rustic border.
07 - Bake for 20 to 25 minutes until the pastry is golden and crisp. Allow to cool slightly before slicing.
08 - Optionally garnish with chopped fresh chives and serve warm or at room temperature.
