Grill patties and onions 5 min. on each side or until burgers are done (160°F) and onions are crisp-tender, brushing onions occasionally with remaining dressing and brushing burgers with barbecue sauce. Top each burger with Singles
; grill 1 to 2 min. or until Singles
begin to melt. Meanwhile, place buns, cut-sides down, on barbecue. Grill until toasted.