Preheat your oven to 400°F (200°C) and grease six 6-ounce ramekins with butter.

Roll out the puff pastry on a lightly floured surface to about 1/8-inch thickness and cut circles slightly larger than the diameter of your ramekins.

In a skillet over medium heat, melt unsalted butter and sauté the diced onion for about 4 minutes until translucent.

Stir in minced garlic and sauté for another 30 seconds until aromatic.

Add mixed mushrooms to the skillet, cook for 8-10 minutes until golden brown.

Sprinkle flour and thyme over the mixture, stir vigorously for about 2 minutes.

Gradually mix in vegetable broth, stir continuously until it thickens and simmers for 3-4 minutes.

Pour in heavy cream, stir, and simmer for another 5 minutes until thick enough to coat a spoon.

Season with salt and pepper, then fill each ramekin with the mushroom mixture, leaving room for pastry.

Top each filling with shredded Gruyère cheese and cover with puff pastry rounds. Seal edges and brush with beaten egg.

Place ramekins on a baking sheet and bake for 15-20 minutes until golden brown.

Allow to cool for about 10 minutes before serving.