The Duke of Wellington was a highly prominent statesman and soldier of the nineteenth century. This dish, however, bears his name not because he was a great gourmet but because the finished joint was thought to resemble one of the brown shiny military boots which were called after him.
Trim and tie up the fillet at intervals with fine string so it retains its shape. Heat the oil and 15g ( 1/2 oz) of the butter in a large frying pan, add the meat and fry briskly on all sides. Press down with a wooden spoon while frying to seal well. Roast for 20 minutes, then set the beef aside to cool and remove the string.
Meanwhile, cook the mushrooms in the remaining butter until soft; leave until cold, then blend with the paté. On a lightly floured surface, roll out the pastry to a large rectangle about 33 x 28 cm (13 x 11 inches) and 0.5 cm ( 1/4 inch) thick. Spread the paté mixture down the centre of the pastry. Place the meat on top in the centre. Brush the edges of the pastry with the egg.
Fold the pastry edges over lengthways and turn the parcel over so that the join is underneath. Fold the ends under the meat on the baking sheet. Decorate with leaves cut from the pastry trimmings. Brush with the remaining egg and bake for 50-60 minutes depending how well done you like your beef, covering with foil after 25 minutes. Allow the Beef Wellington to rest for 10 minutes before serving.