Preheat oven to 200°C (400°F) and grease the inside of six ramekins with butter or cooking spray.

In a large skillet, combine 2 tablespoons of unsalted butter and 1 tablespoon of olive oil over medium heat. Add 1 finely chopped shallot and sauté for about 3-4 minutes until translucent. Stir in 2 minced garlic cloves and cook for another minute.

Add 500 grams of sliced mixed mushrooms to the skillet. Sauté until moisture evaporates, about 5-7 minutes. Season with salt, black pepper, and 1 tablespoon of fresh thyme leaves, cooking for an additional minute.

Sprinkle 2 tablespoons of all-purpose flour over the mushroom mixture and stir well. Cook for 1 minute, then pour in 120 ml of dry white wine and let simmer until reduced, about 3 minutes.

Gradually pour in 240 ml of whole milk, stirring continuously until thickened, about 3-4 minutes. Mix in 100 grams of grated Gruyère cheese and 1 teaspoon of Dijon mustard if desired.

Remove skillet from heat and let cool slightly. Roll out a sheet of thawed puff pastry and cut into six rounds slightly larger than ramekins.

Spoon cooled mushroom filling into greased ramekins. Place pastry rounds on top, sealing edges securely. Cut slits in the top crust for steam to escape.

Brush the tops with a beaten egg for shine and sprinkle with additional thyme leaves. Bake in preheated oven for 18-22 minutes until golden brown.

Let cool for about 5 minutes before serving warm.