In a large skillet over medium heat, melt 2 tablespoons of unsalted butter along with 1 tablespoon of olive oil. Add one finely chopped onion and 2 minced garlic cloves, sautéing until the onion is translucent (about 5 minutes).

Stir in 8 ounces of sliced mushrooms into the pan and cook until golden brown and most of the liquid has evaporated (about 5-7 minutes).

Sprinkle in 2 tablespoons of all-purpose flour, stirring it into the mushroom mixture. Cook for 1-2 minutes and gradually whisk in 1 cup of vegetable broth and 1/2 cup of heavy cream. Let it simmer until thickened (about 5-7 minutes).

Fold in 1 tablespoon of fresh chopped thyme, 1 teaspoon of Dijon mustard, and season with salt and pepper. Add 1 cup of shredded Gruyère cheese, stirring until melted.

Preheat your oven to 400°F (200°C) and grease a 12-cup muffin tin with cooking spray.

Roll out your thawed puff pastry to about 1/8-inch thickness, then cut out circles to fit the muffin cups (you'll need two per pot pie).

Place one circle of pastry into each greased muffin cup and fill with the creamy mushroom mixture. Cover with another pastry circle, sealing the edges tightly.

Brush the tops with an egg wash made from one beaten egg mixed with a splash of water. Optionally score an 'X' on top of each pie for decoration.

Bake for 20-25 minutes, or until puffed and golden brown. Remove from the oven and let cool for 5 minutes before serving.