In medium bowl combine 4 cups ice cold water and 2 teaspoons salt; stir to dissolve salt. Add onion slices; stir to separate rings. Let stand 20 minutes; drain.
Meanwhile, for Sweet-Sour Dressing, in small bowl whisk together vinegar, sugar, and 3/4 teaspoon each salt and freshly ground black pepper. In extra-large bowl combine tomatoes and drained onions. Add dressing; gently toss to coat. Let stand 5 minutes. Serves 6 to 8.