PREHEAT grill to medium-high heat. Mix dressing and 1 tsp. of the barbecue sauce. Stir in tomatoes; set aside.

MIX meat, remaining barbecue sauce and onions. Shape into four (1/2-inch thick) oval patties. Cut baguette crosswise into 4 pieces. Cut each piece lengthwise in half.

GRILL patties 4 min. on each side or until cooked through (160°F). Top each patty with Singles. Add baguettes, cut-sides down, to grill. Continue grilling 1 min. or until baguettes are lightly toasted and Singles are melted. Fill baguettes with cheeseburgers; top with dressing mixture.