Wipe steak with damp cloth; place in large baking dish. Rub with softened butter. Season with salt and pepper; cover with lemon and onion slices.
Combine ketchup, Worcestershire sauce and water, and pour over steak. Cover tightly so meat will steam tender. (Aluminum foil makes a snug-fitting lid.) Bake in moderate oven (350°) 2 hours, or until tender.