Heat the oven to 200⁰C. Melt butter in a large frying pan, cook finely chopped onion for five minutes until it softens. Add the chopped mushrooms, cook for an additional 5 minutes until creamy and soft. Add garlic, parsley, salt and pepper and leave to cool. Once cooled, beat the pate into the mushroom mixture.
Roll out the puff pastry on a lightly floured surface until it is large enough to enclose your beef. Reserve off-cuts for decoration.
Spread the mushroom and pate mixture down the middle of the pastry and lay the beef on top of the mixture. Brush pastry edges with beaten egg and fold the pastry over the meat to enclose it in a neat parcel, pressing firmly to seal the edges.
Lie the meat parcel, seam side down on a baking tray. Decorate with ‘leaf’ or ‘star’ shapes from the left over pastry. Brush entire piece with egg and chill for ten minutes.
Bake for 40 minutes until the pastry is golden and puffed up. Rest for ten minutes before slicing into thick slices and serving with your favourite vegetables.