Heat 2 tablespoons of butter and 1 tablespoon of olive oil in a skillet. Add sliced onions and cook for 20–25 minutes until golden brown.

Preheat the oven to 200°C (400°F). Toss sweet potatoes with olive oil, salt, and pepper, and roast for about 15 minutes.

Roll out chilled puff pastry on a parchment-lined tray to about 1/4-inch thick, leaving a 2-inch border.

Spread caramelized onions over the puff pastry, layer roasted sweet potato slices on top, and crumble feta over it.

Fold the edges of the pastry over the filling, pleating for appearance, and brush edges with beaten egg.

Bake in the preheated oven for 25–30 minutes until golden brown and puffed.