Heat the butter and into it toss the oysters and roughly-chopped mushrooms; cook for 5 min. Transfer to a basin and mix in the breadcrumbs, parsley, lemon rind and seasoning. Stir in the beaten egg. Press the mixture into pocket in steak and sew or skewer edges together. Roast in a warm oven (335° F., Gas 3) to prevent shrinkage for 2 hr. Serve with roast potatoes and pumpkin.