Peel cucumbers; slice paper-thin into a bowl. Sprinkle with salt; cover and refrigerate for 3-4 hours. , Rinse and drain cucumbers. Pat gently to press out excess liquid. In a large bowl, combine cucumbers and onion; set aside. In a small bowl, combine sour cream, vinegar, chives, dill seed, pepper and sugar. , Just before serving, add dressing to cucumbers; toss to coat. Arrange lettuce and tomatoes in a serving bowl if desired. Top with cucumbers.