Trim off the excess fat and season the beef. Melt the butter in a frying pan and sear the beef on all sides. Transfer it to a roasting tin, reserving the fat in the pan, and roast in a preheated oven, 200°C (400°F), Gas Mark 6, for 20 minutes. Leave to cool.
Fry the onions in the pan for 5 minutes while the beef is cooking. Add the mushrooms and a little seasoning and fry until the moisture has evaporated. Add the brandy and fry for a further 1 minute. Leave to cool.
Thinly roll out the pastry to a large rectangle on a lightly floured surface. Spread the top of the meat with the chicken pâté, then press a thick layer of the mushroom mixture over the top. Invert the beef on to the pastry and spread with the remaining mushrooms.
Brush the pastry with beaten egg and bring it up over the fillet to enclose the meat completely, trimming off any bulky areas at the corners. Place, join side down, on a lightly greased baking sheet and brush with more egg. Bake for 35 minutes, until deep golden. Leave to stand for 20 minutes before carving.