Pulse 350g flour, 200g chilled butter, and a pinch of salt in a food processor until it resembles breadcrumbs.
Add the egg yolk and a splash of chilled water, pulsing until the dough forms a ball.
Wrap the dough in cling film and refrigerate for 30 minutes.
Rehydrate dried mushrooms in hot water and chop finely.
Melt 50g butter in a pan over medium heat; add leeks and sauté until soft.
Add sliced mushrooms and minced garlic to the pan, cooking until browned and liquids have evaporated.
Stir in the rehydrated mushrooms, chestnuts, and vegetable stock.
Simmer until the gravy thickens into a glossy consistency.
Roll out the pastry and line your pie dish, reserving enough for a top lid.
Pour the filling into the pastry shell and cover with the remaining dough, sealing the edges.
Bake at 200°C for 50 minutes until the crust is golden brown.