Heat the oil in a large frying pan, add the leeks and mushrooms and fry for 3-4 mins until the leeks have softened. Add the stuffing and cheese. Mix well and set aside.
On a lightly floured surface, roll out the pastry and cut out 2 circles: 1x23cm and 1x25cm. Place the smaller circle on a baking tray lined with baking parchment.
Pile on the filling, leaving a 1.5cm border, then brush the border with the egg. Put the second circle on top to make a lid and crimp the edges to seal them.
Using a sharp knife, score a pattern into the top of the pie, then brush with a little beaten egg to glaze. Leave it in the fridge until you’re ready to cook.
Preheat the oven to 200°C, fan 180°C, gas 6. Bake for 25 mins until well risen and the pastry is golden brown, then serve.