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

Roll out the chilled puff pastry to about 1/8-inch thickness and cut circles slightly larger than your ramekins.

Melt unsalted butter in a skillet over medium heat, sauté diced onion for approximately 4 minutes until translucent, then add minced garlic and cook for an additional 30 seconds.

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

Sprinkle all-purpose flour over the mushroom mixture, stirring to create a roux and cook for 2 minutes.

Gradually pour in vegetable broth while stirring, simmer for 3-4 minutes until thickened.

Stir in heavy cream and simmer for an additional 5 minutes, seasoning with salt and black pepper to taste.

Divide the mushroom filling among ramekins and sprinkle shredded Gruyère cheese on top.

Cover each ramekin with puff pastry circles and press down the edges to seal. Cut small slits in the pastry for steam to escape, and brush with beaten egg.

Bake for 15-20 minutes in the preheated oven until the pastry is golden brown.

Allow to cool for about 10 minutes before serving.