mix meat and 4 Tbsp. barbecue sauce, and 1 1/2 tsp chili powder. Shape into four 1/2-inch-thick patties; set aside.
Brush onions with 2 Tbsp. of the remaining barbecue sauce.
Grill the 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 of the grilling time.
Meanwhile, grill buns, cut-sides down, 1 min. or until lightly toasted.
Fill the buns with burgers, grilled onions, tomatoes, pickles.
top off with some bbq sauce, and stone ground mustard.