Step #2 Pat the beef dry with absorbent paper & spread with 15g (1/2 oz) of the butter.
Step #3 Place in a roasting tin & roast for 40 mins,b asting every once in awhile.
Step #4 Remove & leave until cold.
Step #5 Melt the remaining butter in a saucepan.
Step #6 Add the onion & cook carefully for 5 mins, until softened.
Step #7 Add the mushrooms & cook for about 15 mins until some of the liquid has evaporated.
Step #8 Remove from the heat, season & leave until cold.
Step #9 Increase oven temperature to 230C/450F/Gas8.
Step #10 Roll out the pastry into a rectangle large enough to enclose the beef.
Step #11 Place on a baking sheet.
Step #12 Spread half the mushroom mixture down the centre of the pastry.
Step #13 Place the beef on top, then top this with the remaining mushroom mixture.
Step #14 Brush the edges of the pastry with egg & wrap the pastry round the meat, pressing the long edges together firmly & tucking in the short ends to seal.
Step #15 Use trimmings to make pastry leaves to decorate.
Step #16 Brush all over with beaten egg, then cook in the oven for about 40 mins, cover with foil wrap for the last 10 mins so the pastry stays golden brown.
Step #17 Transfer to a serving dish, garnish & serve.