Preheat the oven to 190°C/375°F/Gas mark 5. Roll out the pastry and line a greased 20cm/8in pie tin.
In a frying pan, sweat the leeks and mushrooms in the butter until tender. Spoon into the pie tin. In a bowl, beat together the egg yolks and crème fraîche. Season with salt and pepper. Pour the mixture onto the leeks.
Bake in the oven for 25–30 minutes until the filling is set. Serve warm.