PREHEAT broiler. Place bread slices on rack of broiler pan. Broil, 4 to 6 inches from heat, 2 minutes on each side or until golden brown on both sides.
MIX dressing and cheese. Reserve 1/4 cup of the dressing mixture. Spread remaining dressing mixture onto one side of the toasted bread slices. Broil, dressing side up, an additional 1 to 2 minutes or until golden brown.
TOSS tomatoes and onions with the reserved 1/4 cup dressing mixture; spoon evenly over bread slices.