Preheat broiler. Place flank steak on a rack in broiler pan and broil 3 inches from heat source until desired doneness is reached, turning once.
Meanwhile, heat oil in a 12-inch skillet over medium heat; add onion slices and cook about 5 minutes. Stir in TABASCO® Sauce, sugar and salt; cook 5 minutes longer until onions are tender, stirring occasionally.
Cut flank steak into thin slices and cut rolls crosswise in half. Top bottom halves of rolls with lettuce leaves, tomato slices, flank steak, and onions; top with remaining roll halves.
Makes 4 servings.