Mix honey mustard and pepper in small bowl. Set aside. Mix ground beef and Sauce Mix-Ins until well blended. Shape into 4 patties.
Grill over medium heat 4 to 6 minutes per side or until burgers are cooked through (internal temperature of 160°F). Grill onion in grill basket or grill rack 3 to 4 minutes or until tender and lightly browned, stirring frequently. Add cheese slices to burgers 1 minute before cooking is completed. Toast rolls on the grill, open-side down, about 30 seconds.
Serve burgers on rolls topped with grilled onions. Serve with honey mustard.