Preheat oven to 425°F (220°C).
Melt the butter in a large, deep skillet or Dutch oven over low heat. Add the leeks, stir, and cover with a lid. Cook for about 5 minutes until soft, stirring occasionally.
Remove the lid, turn the heat up to medium high, and add the mushrooms. Cook for 2 minutes. Add the Quorn chicken pieces and cook for another 2 minutes.
Meanwhile, make a quick gravy: In a small jug or bowl, combine the plain flour and crumbled stock cube. Add a splash of cold water and mix into a smooth paste. Pour in the boiling water and stir to combine.
Add the gravy and crème fraîche to the skillet and stir. Season with salt, pepper, and thyme. Bring the mixture to a boil and let it bubble for 2 minutes to thicken the sauce.
Pour the hot filling into a large oven safe baking dish (about 9x13 inches).
On a lightly floured surface, roll out the puff pastry until it is about 2 inches larger in each direction than your baking dish. Gently score the top of the pastry in a crisscross or diamond pattern.
Brush the edges of the baking dish with a little milk. Place the pastry over the filling, tucking the excess overhang into the sides. Brush the top of the pastry with more milk.
Place the dish on a baking sheet and bake for 20-25 minutes, until the pastry is puffed, golden brown, and the filling is bubbling.
Let the pie rest for 5-10 minutes before serving. Serve with your favorite steamed or roasted vegetables.