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 creme fraiche, stirring continuously to avoid lumps. Simmer until the mixture thickens into a creamy sauce.

Season the mushroom mixture with salt, pepper, chopped thyme, and a squeeze of lemon juice. Adjust seasoning to taste.

Stir in peas and cook for an additional 2-3 minutes until they are heated through.

Transfer the creamy mushroom and pea filling to a baking dish.

Roll out the puff pastry and cover the baking dish with it, trimming any excess. Cut a few slits on top to allow steam to escape.

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

While the pie is baking, roast potatoes in the oven until golden brown and crispy.

Remove both the pie and roast potatoes from the oven.

Serve the Mushroom and Pea Pie with a side of roast potatoes. Enjoy your delicious and comforting meal!