Heat butter and oil in a large heavy based frying pan until butter is bubbling. Add eschalots and mushrooms, cook until softened and juice released from the mushrooms has evaporated. Season to taste with salt and pepper and transfer to a bowl to cool.
Season beef with salt and pepper, reheat frying pan and sear beef on all sides until well browned. Add brandy to pan and flambe. Wait for flames to die down and set beef aside to cool.
To assemble, lay one sheet of pastry on a baking sheet. Spread half of the mushroom mixture over the pastry to fill a 10cm x 20cm rectangle. Spread half the pate over one side of the beef and place it pate side down on top of the mushroom mixture. Spread the top of beef with remaining pate and top with remaining mushrooms.
Place second sheet of pastry over the beef and ease it down around the sides of the beef. Press pastry down around the edges to stick to the first sheet. Trim edges so a neat rectangle shaped parcel is formed. Using a fork, press the edges of the pastry together.
If you like, cut remaining pastry scraps into pretty designs and place on top of the pastry parcel. (Can be prepared ahead to this stage and refrigerated. Remove from fridge at least 2 hours before cooking for meat to return to room temperature).
Preheat oven to 200oC (180c fan forced). Piece pastry with a fork a couple of times to allow steam to escape during cooking. Brush with egg yolk mixture.
Bake in a preheated oven for 40 minutes to 1 hour or until pastry is golden brown and beef is cooked as desired. Rest beef wellington for 10 minutes once removed from oven to allow juices to set.