Trim ends off cucumber; cut lengthwise, then crosswise in half. Run spoon down centers to remove seeds. Use sharp vegetable peeler or other slicer to cut cucumbers into thin ribbons; place in large bowl of ice water. Refrigerate 30 min.
Crumble Noodles into large bowl. Discard Seasoning Packet from soup mix or reserve for another use.
Drain cucumbers.
Add to noodles along with the lettuce and onions; mix lightly.