Preheat broiler. Place tomato halves on a broiler pan coated with cooking spray. Broil 10 minutes or until blackened. Cool 10 minutes; peel and core. Combine roasted tomato, basil, and next 6 ingredients (basil through pepper) in a blender; process until smooth.
Steam beans, covered, 7 minutes or until crisp-tender; rinse with cold water, and drain. Place 1 cup lettuce on each of 4 plates. Arrange beans, cherry tomatoes, and onion over lettuce. Spoon 3 tablespoons tomato dressing over each salad.