PREHEAT barbecue to medium-high heat. Mix meat and 2 Tbsp. of the barbecue sauce. Shape into four 1/2-inch-thick patties; set aside. Brush onions with 1 Tbsp. of the remaining barbecue sauce.

GRILL patties and onions 4 min. on each side or until burgers are cooked through (160ºF) and onions are crisp-tender, brushing burgers with remaining barbecue sauce and topping with the Singles for the last minute of the grilling time. Meanwhile, grill buns, cut-sides down, 1 min. or until lightly toasted.

FILL buns with the burgers, onions and tomatoes.