1. Combine red and yellow tomatoes, cucumber, and onion in a large bowl.
2. Combine vinegar and next 7 ingredients in a small bowl; stir with a whisk until combined. Pour over tomato mixture, stirring gently to coat. Cover and chill 1 hour.
3. Divide shredded lettuce among 4 salad plates; spoon tomato mixture over lettuce.
carbo rating: 8