Sauté mushrooms in butter until just tender, stir in porcini, 80g of the chestnuts and parsley, season well and allow to cool. Meanwhile preheat oven to 200ºC (180ºC for fan assisted ovens)/400ºF/Gas M6.
Unroll one pastry sheet onto a lined baking tin, stir the soured cream into the cooled mushroom mixture and pile onto centre of sheet, leaving a border all round edges. Brush border with beaten egg.
Place second pastry sheet on top and press edges together well to form a seal and crimp up the edges to give a rustic appearance. Brush top again with beaten egg and sprinkle with remaining chestnuts – roughly chopped.
Make a couple of slits in top and bake for approx. 20 mins until pastry golden brown. Serve hot.