Melt margarine in saucepan. Add chopped onions and saute until tender. Add tomato puree, brown sugar, Worcestershire sauce, lemon juice, mustard and salt. Mix well. Simmer over moderate heat 30 - 40 minutes. Add cooked beef or ham slices. Heat thoroughly 10 - 15 minutes. Meanwhile cut 10 buns in half; toast and butter lightly. Serve barbequed beef or pork on buns, either open-faced or as a closed sandwich.