Heat the oven to 200c/fan 180c/gas 6. Cover the pastry with a large circle of baking paper and fill the centre with baking beans or uncooked pulses or rice. Bake for 15 minutes then remove the paper and beans and cook for a further 7-10 minutes, until very pale golden. Take out and turn the oven down to 160c/140c/gas 3.
Melt the butter in a large frying pan and add the leeks and a pinch of salt. Cover with a lid and cook over a low-medium heat for 5 minutes. Remove the lid, turn the heat up and add the mushrooms. Cook briskly for a few minutes, stirring often. Season with a little salt and lots of black pepper.