Trim all fat from the fillet of beef and heat the butter in a pan. Brown the fillet of beef in the butter put fillet of beef aside and let it go cold.
Finely slice the mushrooms and cook in a little oil. mix mushrooms and pate together and coat the fillet of beef with the pate and mushrooms.
Roll out pastry very thinly to give a piece 1 1/4 times the length of the fillet. And warp the pastry around the fillet.
Seal all the edges with egg yolk and brush the pastry lightly with the rest of the egg yolk.
Put on a baking sheet and cook for 45 minutes in a moderate oven.
If the pastry starts to brown too quickly lightly cover with foil until nearly cooked then remove to finish browning. Serve with fresh Vegetables and potatoes.