In a medium skillet, melt 2 tablespoons of unsalted butter with 1 tablespoon of olive oil over medium heat. Add 1 finely chopped onion and 2 minced garlic cloves, sautéing for about 4-5 minutes until the onion becomes translucent and fragrant.

Incorporate 12 ounces of sliced mixed mushrooms into the skillet, stirring well. Cook for 6-8 minutes until the mushrooms are golden brown and their moisture has evaporated.

Sprinkle 2 tablespoons of all-purpose flour over the mushroom mixture, stirring constantly for about 1 minute. Gradually pour in 1 cup of vegetable or chicken broth and 1/2 cup of heavy cream, whisking continuously until the mixture thickens.

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. Add in 1 cup of grated Gruyère cheese, mixing until melted.

Preheat your oven to 400°F (200°C) and grease a muffin tin or individual ramekins.

Roll out store-bought puff pastry and cut out circles to fit into your muffin cups. Place a circle in the bottom of each muffin cup and fill with the creamy mushroom mixture. Cover with another pastry circle and seal the edges tightly.

Brush the tops of the pot pies with an egg wash made from 1 beaten egg mixed with a splash of water. Bake for about 20-25 minutes, until the tops are golden brown.

Let the pot pies cool for a few minutes, then carefully take each pie out and serve warm.