Step 1: In a large skillet, melt 1 tablespoon of butter and add 1 tablespoon of olive oil over medium-low heat. Sauté thinly sliced onions and a pinch of salt for 20-25 minutes until caramelized.

Step 2: Preheat your oven to 200°C (400°F). Slice the sweet potatoes and toss them with 1 tablespoon of olive oil, salt, and pepper. Roast for about 15 minutes until tender and slightly caramelized.

Step 3: On a lightly floured surface, roll out the puff pastry into a large circle about ¼ inch thick on a baking sheet lined with parchment paper.

Step 4: Spread the caramelized onions evenly in the center of the puff pastry, layer with roasted sweet potatoes and crumbled feta, then sprinkle fresh thyme on top.

Step 5: Fold the edges of the puff pastry over the filling, making pleats. Brush the exposed pastry with a beaten egg.

Step 6: Bake for 25-30 minutes at 200°C (400°F) until the pastry is puffed and golden brown.

Step 7: Let cool slightly before serving. Garnish with fresh herbs and serve warm as a main dish or appetizer.