# What You'll Need:
→ Pastry
01 - 1 sheet frozen puff pastry, thawed (8.8 oz)
→ Onions
02 - 3 large yellow onions, thinly sliced
03 - 2 tbsp unsalted butter
04 - 1 tbsp olive oil
05 - 1 tsp sugar
06 - 1/2 tsp salt
07 - 1/4 tsp freshly ground black pepper
→ Filling & Topping
08 - 1/2 cup crème fraîche or sour cream (4 fl oz)
09 - 1 tsp Dijon mustard
10 - 1/2 cup grated Gruyère cheese (2.1 oz)
11 - 1 tsp fresh thyme leaves or 1/2 tsp dried thyme
12 - Optional: 1 tbsp chopped fresh chives for garnish
# Directions:
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 frequently, until deeply golden and caramelized, about 25 to 30 minutes.
03 - Roll out the puff pastry on a lightly floured surface into a 12-inch square or circle. Transfer to the prepared baking sheet.
04 - Combine crème fraîche and Dijon mustard in a small bowl. Spread evenly over the pastry, leaving a 1-inch border.
05 - Scatter half the Gruyère over the crème mixture, top with caramelized onions, sprinkle thyme, and finish with remaining Gruyère.
06 - Fold the pastry edges gently over the filling to create a rustic crust.
07 - Bake for 20 to 25 minutes until the pastry is golden and crisp. Let cool slightly before slicing.
08 - Optionally garnish with chopped fresh chives. Serve warm or at room temperature.
