Preheat the oven to 375°F (190°C).

In a large pan, heat olive oil over medium heat. Add finely chopped onions and sauté until they become translucent.

Add sliced mushrooms to the pan and cook until they release their moisture and become golden brown.

Stir in flour to create a roux, cooking for 2-3 minutes to remove the raw flour taste.

Gradually pour in vegetable stock and milk, stirring continuously to avoid lumps. Simmer until the mixture thickens into a creamy sauce.

Season the mushroom mixture with salt, pepper, and chopped thyme. Adjust seasoning to taste.

Roll out the pie crust and line a pie dish with it. Pour the creamy mushroom filling into the crust.

Cover the pie with a second layer of rolled-out pie crust, sealing the edges. Cut a few slits on top to allow steam to escape.

Bake in the preheated oven for 30-35 minutes or until the crust is golden brown and the filling is hot and bubbly.

Remove from the oven and let the Creamy Mushroom Pie cool slightly before serving.

Slice and serve the creamy mushroom pie, optionally garnished with additional thyme. Enjoy your comforting and flavorful meal!