Caramelize the Onions: In a large skillet, melt 1 tablespoon of butter with 1 tablespoon of olive oil over medium-low heat. Add thinly sliced onions with a pinch of salt and cook for about 20-25 minutes until golden brown.

Roast the Sweet Potatoes: Preheat your oven to 200°C (400°F). Toss sliced sweet potatoes with olive oil, salt, and freshly ground black pepper, then roast for 15 minutes until tender.

Prepare the Puff Pastry: Unroll the cold puff pastry onto a parchment-lined baking tray. Spoon caramelized onions into the center, leaving a 2-inch border.

Assemble the Galette: Layer roasted sweet potatoes over onions, crumble feta on top, and sprinkle with thyme. Fold the pastry edges over the filling, then brush with an egg wash.

Bake the Galette: Place in the oven and bake for 25-30 minutes until the pastry is puffed and golden brown.