Pre-heat the oven to 220°C/425°F/Gas Mark 7. Trim and tied up the beef at intervals with fine string so it retains its shape. Heat the oil and 15g of the butter in a large frying pan, add the beef and seal and lightly colour on all sides.
Roast for 20 minutes, allow the beef to cool then remove the string. Fry the sliced mushrooms in the remaining butter until soft, allow to cool and mix with the paté.
On a lightly floured surface, roll out the pastry into a large rectangle to a thickness of 1/2 inch. Spread the paté and mushroom mixture along the centre of the pastry. Place the meat on top in the centre. Brush with beaten egg.
Fold the pastry edges over and turn over so that the join is underneath, folding the ends under the meat. Place on a baking tray. Decorate with leaves cut from the pastry trimmings, brush with the remaining egg.
Bake for 50-60 minutes, covering with foil after 25 minutes. Allow to rest for 10 minutes before serving.