Cut meat into 1/2 in strips, remove all fat. (The butcher will do this at no charge, if you ask.) Pat dry steak strips with paper towel; sprinkle with salt and pepper. Melt 1 Tbsp. Diet margarine in non-stick frying pan, add strips of meat, cook and stir until brown on both sides. Remove meat from pan and set aside. Saute onions in same pan. Saute mushrooms until almost tender. Return meat and onions to pan. Combine beef broth, mustard, bay leaf and tomato paste and pour over meat, onions and mushroom. Cover and let simmer gently for 1/2 hour, or until meat is tender. Remove bay leaf. Make smooth paste of flour and water in a cup, stir into frying pan and cook over low heat, stirring constantly until mixture thickens. Add non-fat yogurt and heat through. Do Not Boil. Delicious with noodles or rice.