The Preparation
Preheat oven to 375-400°F [approx].
Thaw puff pastry and roll out slightly larger (about an inch bigger all around). Place on a parchment-lined sheet pan.
Score a border about 1 inch from the edges with a knife — this allows the border to puff up.
Heat butter and olive oil in a pan. Add sliced onions with salt and pepper. Cook to begin caramelizing but do not fully caramelize (they will continue cooking in the oven).
Add sliced mushrooms to the pan (add after onions so they don't release liquid and prevent caramelization). Cook until softened.
Add minced garlic in the last minute and cook until fragrant. Remove from heat.
Spread grated cheese within the scored border of the pastry.
Top with the sauteed mushroom-onion mixture.
Sprinkle generously with fresh thyme leaves.
Optionally brush the border with egg wash for extra browning.
Bake for 15-20 minutes until puff pastry is golden and crisp, and cheese is melted and bubbly.
Let cool slightly before slicing.
