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

Roll out the puff pastry until 1/8-inch thick and cut circles slightly larger than the ramekins.

Melt 2 tablespoons of unsalted butter in a skillet over medium heat, add the chopped onion and sauté for 4 minutes.

Stir in minced garlic and mixed mushrooms; cook for 8-10 minutes until golden-brown.

Sprinkle 2 tablespoons of all-purpose flour over the mixture, stir, and cook for 2 minutes to form a roux.

Gradually pour in 1 cup of vegetable broth, stirring to avoid lumps; bring to a gentle simmer.

Add 1/2 cup of heavy cream and stir until creamy; season with salt and pepper to taste.

Divide the filling evenly among the ramekins, topping each with grated Gruyère cheese.

Cover with puff pastry rounds, sealing edges; crimp with a fork and brush with egg wash.

Bake on a baking sheet for 15-20 minutes until golden; cool for 10 minutes before serving.