Grill meat patties and onions 5 min. on each side or until burgers are done (160°F) and onions are crisp-tender, brushing onions with remaining barbecue sauce for the last minute. Remove onions from barbecue. Top burgers with cheese; grill 1 to 2 min. or until melted. Meanwhile, place buns, cut sides down, on barbecue grate; grill until toasted.