1. Preheat oven to 400°F (200°C). Line a baking sheet with parchment paper.

2. In a large skillet, heat butter and olive oil over medium-low heat.

3. Add sliced onions, salt, sugar, and pepper. Cook slowly, stirring occasionally, for 25–30 minutes until onions are golden brown and caramelized.

4. Stir in thyme and balsamic vinegar (if using), then remove from heat and let cool slightly.

5. On a lightly floured surface, roll out puff pastry into a 10×12-inch rectangle. Transfer to the prepared baking sheet.

6. Use a knife to score a 1-inch border around the edges (do not cut all the way through).

7. Brush the border with egg wash.

8. Sprinkle 1/2 cup of Gruyère cheese over the base (inside the border).

9. Spread caramelized onions evenly over the cheese, then top with remaining Gruyère.

10. Bake for 20–25 minutes, or until pastry is puffed and golden brown.

11. Cool slightly before slicing.

12. Garnish with fresh thyme or microgreens and serve warm or at room temperature.