Steps
1. Preheat the oven to 400°F (200°C) and line a baking sheet with parchment paper.
2. In a large skillet, melt the butter and olive oil over medium heat. Add the sliced onions and cook until they start to soften, about 5 minutes.
3. Sprinkle the sugar over the onions and continue cooking, stirring occasionally, until the onions are caramelized and golden brown, about 20 minutes. Season with salt and pepper.
4. Roll out the puff pastry sheet on a lightly floured surface and transfer it to the prepared baking sheet. Prick the pastry with a fork to prevent it from puffing up too much.
5. Spread the caramelized onions evenly over the puff pastry, leaving a small border around the edges. Crumble the goat cheese over the onions.
6. Bake in the preheated oven for 20-25 minutes, or until the pastry is golden brown and the cheese is melted and bubbly.
7. Remove from the oven and let cool slightly. Garnish with fresh thyme leaves before serving.