Preheat broiler. Spray medium skillet with cooking spray; heat over medium heat. Add franks; heat 5 minutes or until browned, turning occasionally. Meanwhile, toast rolls on baking sheet under broiler.
Stir together ketchup, mustard and brown sugar in small bowl; set aside. Add peppers and onions to skillet; cook and stir 6 minutes or until hot.
Spread ketchup mixture on toasted rolls. Place franks in rolls and top each with peppers and onions. Drizzle with additional mustard, if desired. Serve immediately.