Pare the cucumbers and slice them as thin as possible. If the cucumbers are old enough to have large seeds in the middle, cut that section in two and spoon out the seeds. Cut the onion in thin slices. Sprinkle the vegetables with salt and pepper and marinate them in vinegar for at least 8 or 4 hours in the refrigerator. Drain the salad well and dress it with oil and, if you wish, sour cream. Arrange in a serving dish and return to the refrigerator so that the salad will be icy cold at serving. Finely minced parsley, chives, or mint can be used to lend variety to this dish.