1. Heat grill to medium-high. Combine tomatoes, dressing and 1/2 tsp. sauce. Cut baguette lengthwise in half, then cut crosswise into four pieces.
2. Mix meat, onions and remaining steak sauce. Shape into four (1/2-inch thick) oval patties.
3. Grill 4 min. on each side or until cooked through (160°F); top with Singles. Add baguettes, cut-sides down, to grill. Grill 1 min. or until baguettes are lightly toasted and Singles are melted. Fill baguettes with cheeseburgers; top with tomato mixture.