Place 55g/2oz of the steak into a food processor along with the mushrooms, garlic and cream and pulse to a smooth paste.
Roll out the pastry until it is large enough to wrap the remaining piece of steak, and spread half of the steak and mushroom mixture over.
Brush the rest of the steak with the olive oil and sear in a hot chargrill pan for 1-2 minutes on both sides, or until golden-brown. Place onto the pastry and spread the remaining steak and mushroom mixture over the meat.
Bring one end of the pastry over the meat to cover and seal the ends. Brush the pastry parcel all over with the beaten egg and place onto a baking sheet.
Bake the parcel in the oven for 12-15 minutes, or until the pastry is golden-brown and crisp.
Serve the beef Wellington in slices.