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HEAT grill to medium-high heat.
MIX meat and 2 Tbsp. barbecue sauce; shape into 4 (1/2-inch-thick) patties. Brush onions with 1 Tbsp. of the remaining barbecue sauce.
GRILL patties and onions 4 min. on each side or until burgers are done (160ºF) and onions are crisp-tender, brushing burgers with remaining barbecue sauce and topping with the 2% Milk Singles for the last minute. Meanwhile, grill buns, cut-sides down, 1 min. or until lightly toasted.
FILL buns with burgers, onions and tomatoes.