Steps
1. Preheat the oven to 400°F (200°C).
2. In a pan, melt butter and olive oil over medium heat. Add the sliced onions and cook, stirring occasionally, until caramelized and golden brown, about 20-25 minutes.
3. Roll out the puff pastry on a baking sheet and prick the surface with a fork.
4. Spread the caramelized onions over the pastry, leaving a border around the edges.
5. Bake in the preheated oven for 15-20 minutes or until the pastry is golden brown.
6. Garnish with fresh thyme leaves before serving.