In a large skillet, heat 2 tablespoons of butter and 1 tablespoon of olive oil over medium heat. Add thinly sliced onions and a pinch of salt, stirring occasionally for about 20-25 minutes until caramelized.

Preheat your oven to 400°F (200°C). Toss sliced sweet potatoes with olive oil, salt, and pepper. Spread in a single layer on a baking sheet and roast for about 15 minutes.

Roll out a chilled sheet of puff pastry on a lightly floured surface to about 12 inches in diameter. Transfer to a parchment-lined baking sheet.

Spread caramelized onions evenly over the pastry. Layer roasted sweet potatoes on top, followed by crumbled feta cheese and sprinkles of fresh thyme.

Fold the edges of the puff pastry over the filling, pleating to create a rustic look. Brush exposed pastry with a beaten egg.

Bake for 25-30 minutes or until the pastry is puffed and golden brown.

Let the galette cool slightly before slicing. Serve warm with a drizzle of olive oil and fresh herbs on top.