Heat the oven to 300°F (150°C) gas mark 2.
1. Put a large piece of foil on a baking sheet or heatproof dish and sprinkle half of the soup mix onto it.
2. Place the steak on top, and sprinkle over the remaining soup mix.
3. Dot with small pieces of butter and loosely seal the foil.
4. Cook undisturbed in the oven for about 3 hours.
5. Serve either straight from the foil or lift it out onto a serving dish.
6. Spoon any juices in the foil over the meat.
Accompaniments: Serve with a rich onion gravy or sauce or, if preferred, a thin gravy. Jacket potatoes and braised celery or carrots cooked beside the beef in the oven will make this a complete oven meal.