1. Cut the onions into thin rings. Peel, deseed and dice the tomatoes. Bring a little salted water to the boil in a small pan. Cut the bone marrow into four equal slices, place in a sieve and boil in the water for 3 minutes, then leave to cool.
2. Heat 1 tablespoon of oil in a large frying pan and fry the onions until golden brown. Add the tomatoes and cook for a further 5 minutes, then remove the onions and tomatoes from the pan and keep them warm.
3. Fry the steaks in the remaining oil for 3 minutes each side, transfer them to warm plates and season to taste. Heat the cocktail onions in the juice in the pan. Spoon the marrow and cocktail onions over the steaks and top with the onion and tomato mixture.
4. Heat the butter in the frying pan, then sprinkle it over the steaks with the parsley.
Serve with: noodles or parsley potatoes and green salad.