Begin by washing the cucumbers and tomatoes under cool running water. If the cucumbers have a thick or waxy skin, you can peel them, but if you're using English or Persian cucumbers, the skin is usually tender and fine to leave on. Slice the cucumbers into rounds or half-moons, whichever you prefer. Then chop the tomatoes into small, bite-sized pieces—if you’re using cherry or grape tomatoes, just slice them in half. Peel and thinly slice half of a small red onion. For a milder onion flavor, you can soak the slices in cold water for 5–10 minutes, then drain them well before using.