PREHEAT barbecue to medium-high heat. Mix meat, onions and dressing. Shape into four 1/2-inch-thick patties.
GRILL patties 4 min. on each side or until medium doneness (160°F). Top with the Singles. Place buns, cut-sides down, on grate of barbecue along with the burgers. Grill 1 min. or until buns are lightly toasted and Singles are melted.
MIX ketchup and seasoning. Fill buns with the cheeseburgers; top with the ketchup mixture.
serving size = 1 burger