Preheat grill to medium-high heat.
Mix meat and 2 Tbsp. of the bbq sauce. Shape into four 1/2-inch thick patties; set aside.
Brush onions with 1 Tbsp. of the remaining bbq sauce.
Grill patties and onions 4 minutes on each side or until burgers are cooked through (160°F) and onions are crisp-tender, brushing burgers with remaining bbq sauce and topped with the cheese for the last minute of the grilling time.
Meanwhile, grill buns, cut-side down, 1 minute or until lightly toasted.
Fill buns with burgers, onions and tomatoes.