HEAT grill to medium heat.
COMBINE tomatoes, dressing and 1/2 tsp. steak sauce.
MIX meat, onions and remaining steak sauce; shape into 4 (1/2-inch-thick) oval patties.
GRILL patties 4 to 5 min. on each side or until done (160°F); top with 2% Milk Singles. Add bread, cut-sides down, to grill. Grill 1 min. or until Singles are melted and bread is lightly toasted.
FILL bread with cheeseburgers; top with tomato mixture.