In a large skillet over medium heat, melt 2 tablespoons of unsalted butter with 1 tablespoon of olive oil. Add 1 finely chopped onion and 2 minced garlic cloves, sautéing until the onions are translucent, about 3-4 minutes.

Stir in 8 ounces of sliced mushrooms, cooking until they are golden and their moisture has evaporated, approximately 6-8 minutes.

Sprinkle 2 tablespoons of all-purpose flour over the sautéed mushrooms, stirring for about a minute to cook off the raw flour taste.

Gradually whisk in 1 cup of vegetable broth and 1/2 cup of heavy cream, allowing the mixture to simmer. Cook until thickened, around 3-5 minutes.

Add 1 tablespoon of fresh thyme, Dijon mustard (if using), and seasoning with salt and pepper. Finally, fold in 1 cup of grated Gruyère cheese until it melts beautifully into the filling.

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

Roll out 1 sheet of puff pastry on a floured surface and cut out circles that are slightly larger than the muffin cups.

Place each pastry circle into the muffin tin, ensuring they fit securely, then fill each cup generously with the creamy mushroom mixture.

Cover the filled cups with smaller pastry circles, pressing the edges together to seal the pot pies tightly.

Brush the top of each pastry with an egg wash made from 1 beaten egg for a golden finish.

Optionally, sprinkle with herbs or additional cheese for extra flair before placing the muffin tin in the preheated oven.

Bake the Mini Mushroom & Gruyère Pot Pies in the oven for 20-25 minutes, or until the pastry is puffed and golden brown.

Remove them from the oven and let them cool slightly before enjoying the warm comfort of these lovely pot pies.