TRIM ends off a cucumber and cut in half crosswise and lengthwise. Run a spoon down the center to remove seeds. Using a sharp vegetable peeler or other slicer, thinly slice each piece lengthwise into ribbons. Place ribbons in a large bowl of ice water and chill at least 30 minutes.
DRAIN and toss cucumbers in large bowl with lettuce and onions.
DIVIDE salad among 4 bowls and drizzle with dressing. For added crunch, crumble a package of dry ramen noodles into salad before tossing. (Reserve seasoning packet from noodles for another use.)