Begin by heating 2 tablespoons of unsalted butter and 1 tablespoon of olive oil in a large skillet over medium heat. Once melted, add 1 chopped onion and 2 minced garlic cloves, sautéing them for about 3-4 minutes until aromatic and slightly golden.

Add 8 ounces of sliced mushrooms to the skillet, stirring well. Cook for 6-8 minutes, allowing the mushrooms to become golden brown and lose their moisture.

Sprinkle 2 tablespoons of all-purpose flour over the mushroom mixture, stirring to combine for about 1 minute. Gradually pour in 1 cup of vegetable or chicken broth and ½ cup of heavy cream, whisking constantly. Simmer for 4-5 minutes until the mixture thickens and becomes creamy.

Remove the skillet from the heat and stir in 1 tablespoon of fresh thyme, 1 teaspoon of Dijon mustard, and salt and pepper to taste. Fold in 1 cup of grated Gruyère cheese until melted and creamy.

Preheat your oven to 400°F (200°C). Roll out a sheet of puff pastry on a lightly floured surface and cut it into squares that fit a muffin tin. Press the pastry into the muffin cups, leaving enough overhang. Fill each pastry with the mushroom mixture, then cover with additional pastry, sealing the edges.

Brush the tops of the pot pies with an egg wash for a golden finish. Place the muffin tin in the preheated oven and bake for 20-25 minutes, or until the pastries are puffed and golden brown.