Caramelized Onion and Gruyère Tart

1 sheet puff pastry (thawed if frozen)

2 medium onions, thinly sliced

1 tbsp unsalted butter

1 tsp olive oil

Pinch of salt

1/2 cup Gruyère cheese, shredded

1 large egg, beaten (for egg wash)

1 tsp fresh thyme leaves

1 tbsp aged balsamic vinegar

Preheat your oven to 375°F (190°C). Roll out the puff pastry slightly and cut into a 6-inch circle or square for two tarts. Place on a parchment-lined baking sheet and prick the centers with a fork, leaving a 1/2-inch border.

In a skillet, heat butter and olive oil over medium-low heat. Add onions and salt, cooking for 20-25 minutes until deeply caramelized, stirring occasionally. Spread the caramelized onions evenly over the pastry, leaving the border. Sprinkle with Gruyère and thyme. Brush the borders with the beaten egg and bake for 15-18 minutes, until golden. Finish with a drizzle of aged balsamic and serve warm.