Preheat greased grill to medium heat. Place onions in center of 18X12-inch sheet of heavy-duty foil. Top with 1 Tbsp. of the barbecue sauce. Bring u foil sides. Double fold top and ends to seal packet, leaving room for heat circulation inside; set aside. Mix meat, peppers and 1 Tbsp. of the remaining barbecue sauce; shape into four patties.
Grill patties and foil packet 14 to 18 min. or until patties are cooked through (160 degrees), turning patties over after 8 min. and brushing occasionally with the remaining barbecue sacue. Remove foil packet after 15 min. Add buns, cut-sides down, to grill. Cook 2 min. or until toasted.
Cut slits in packe to release steam before opening packet. Spread bottom halves of buns with dressing; cover with burgers, cheese, onion mixture and tops of buns.
Makes 4 servings, 1 burger each.