Pre-heat oven to 425° F., gas mark 7.
Trim fat and skin from fillet. Roll out pastry very thinly to give a piece 1¼ times the length of your fillet and wide enough to wrap round it with a good overlap. Spread the fillet all over with slightly softened butter. Lightly spread some pate where the meat is to lie along the middle of the pastry and lay a line of mushrooms, caps downwards, alo