In 3 1/2- to 6-quart slow cooker, combine onions and margarine. Cover; cook on high setting for 30 to 35 minutes or until onions begin to slightly brown around edges.
In small bowl, blend flour, sugar, pepper and Worcestershire sauce until smooth. Stir flour mixture and broth into onions.
Cover; cook on low setting for 7 to 9 hours or on high setting for 3 to 4 hours.
About 10 minutes before serving, place bread slices on broiler pan. Sprinkle with mozzarella and Parmesan cheeses. Broil 5 to 6 inches from heat for about 3 minutes or until cheese is melted. Top individual servings of soup with 1 slice cheesy bread.