Pat the
beef dry with kitchen paper and spread with 15
g/1/2
oz of the butter. Place in a roasting tin and
roast in a preheated oven, 190°C (375°F),
Gas Mark 5, basting occasionally, for 40
minutes. Remove and cool.
Melt the remaining
butter in a saucepan. Add the onion and cook
gently for 5 minutes
until softened. Add the mushrooms and cook for
about 15 minutes until most of the liquid has
evaporated.
Remove from
the
heat,
season to taste with salt and pepper and
leave until cold. Rollout the pastry to a rectangle large enough to
enclose the fillet of beef.
Carefully place the
pastry on a baking sheet. Spread half the
mushroom mixture down the centre of the pastry.
Place the beef on top, then top
with the remaining mushroom mixture.
Brush the
edges of the pastry with egg and wrap the pastry
around the meat, pressing the long edges
together firmly and tucking in the short ends to
seal. Use the trimmings to make pastry leaves to
decorate.
Brush the pastry allover with beaten
egg, then cook in a preheated oven, 230°C
(450°F), Gas Mark 8, for about 40 minutes; cover
with foil for the last 10 minutes so the pastry
stays golden brown.
Transfer to a serving dish,
garnish with watercress sprigs, then slice and
serve.