Preheat your oven to 200°C (400°F) and grease six ramekins with butter or oil.

In a skillet, heat unsalted butter and olive oil, then sauté shallot until translucent, adding garlic for another minute.

Add sliced mushrooms to the skillet and sauté until browned, seasoning with thyme, salt, and pepper.

Sprinkle flour over the mixture and stir for a minute, then pour in white wine and simmer until reduced.

Stir in whole milk until thickened, then add grated Gruyère and Dijon mustard.

Roll out puff pastry on a floured surface and cut out rounds larger than ramekins.

Fill each ramekin with mushroom mixture, cover with pastry, sealing the edges and cutting slits for steam.

Brush pastries with beaten egg and sprinkle thyme leaves on top.

Bake on a sheet for 18-22 minutes, until golden brown and puffed up.

Let pies cool for 5 minutes before serving warm.