Preheat your oven to 400°F (200°C).

In a large skillet over medium heat, sauté the onion in a splash of oil for about 3–4 minutes until soft. Then add garlic and mushrooms, cooking for an additional 5–7 minutes until tender.

Stir in thyme, salt, and pepper. Add cream and simmer for 2–3 minutes until thickened. Remove from heat and fold in Gruyère cheese until melted.

Roll out puff pastry to about 1/8 inch thick. Cut circles slightly larger than muffin tin cups and press them into the tin.

Fill the pastry-lined cups with mushroom and Gruyère filling until about three-quarters full.

Cut additional circles from puff pastry for tops. Seal edges by crimping with a fork.

Brush the tops with beaten egg for a golden sheen.

Bake for 18–20 minutes until golden brown.

Cool in the muffin tin for 5 minutes before serving warm.