In a large skillet, heat 2 tablespoons of unsalted butter and 1 tablespoon of olive oil over medium heat. Add 1 finely chopped onion and 2 minced garlic cloves, sautéing for 3-4 minutes until fragrant. Stir in mixed mushrooms for 6-8 minutes until golden brown. Sprinkle in 2 tablespoons of all-purpose flour, stirring constantly for 1 minute, then gradually whisk in 1 cup of vegetable broth and 1/2 cup of heavy cream. Simmer for 5-7 minutes until thickened.

Remove the skillet from heat, stir in the fresh thyme, 1 tablespoon of Dijon mustard (if using), salt, and pepper to taste. Fold in 1 cup of shredded Gruyère cheese until melted and smooth. Set aside to cool slightly.

Preheat your oven to 400°F (200°C). Grease a muffin tin or ramekins with cooking spray or butter. Roll out puff pastry on a floured surface.

Cut circles from the rolled-out puff pastry to fit into the muffin cups. Place each circle into the greased muffin tin and spoon in the mushroom filling. Add another pastry circle on top, sealing the edges by crimping with a fork. Brush the tops with an egg wash.

Bake for 20-25 minutes until the pastry is puffed and golden brown. Watch carefully as baking times may vary.

Remove from the oven and cool for about 5 minutes. Gently take each pie out of the muffin tin and place them on a serving platter. Enjoy warm!