Trim the beef fillet and season with plenty of salt and pepper. Heat the butter in a large frying pan and sear the fillet over a high heat turning with 2 spoons until brown on all sides. Warm the brandy, set alight and pour over the beef. Shake the pan until the flames subside. Remove to a dish and allow to cool. Reserve any liquid remaining in the frying pan for the sauce. Spread the pate on top of the beef. Slice the mushrooms and cook in a little butter for a few minutes. Arrange them on the pate. Roll out puff pastry on a lightly floured board to a 3mm thickness. Trim the edges, place fillet on one side of the pastry. Brush edges with beaten egg. Fold the other half of the pastry over the fillet. Press the edges of the pastry together and tuck underneath to form a neat parcel. Place on a baking sheet, which has been dampened with water. Cut thin strips of pastry, about 5mm wide. Brush the pastry encasing the fillet with egg and then arrange the pastry strips in a lattice fashion (about 25 mm apart). Brush with beaten egg. Bake in a very hot oven for 15 minutes, reduce to moderate and cook for further 15 - 20 minutes.