Season beef with salt & pepper, fry in hot oil until browned all over. Place beef in a roasting tin and cook in a preheatd oven 220°C (425°F / gas 7) for 25 mins rare, 30 mins medium or 40 mins well done. Leave to cool.
Melt the butter and add the onions and mushrooms. Cook until soft and the moisture has evaporated. Leave to cool.
Add the pate to the onion and mushrooms mixture, season with salt & pepper and stir well. Cover the beef with the pate mixture and then wrap beef in the pastry. Brush pastry with beaten egg and cook for 40-45mins at 220°C (425°F / gas 7).
Leave to stand for 10mins