I figure everyone's mom made a salad something like this when they grew up. Well mine did, and it's a favorite to this day, but I have taken it a bit further in that I often make it in a big chunk style to eat with one's hands for hands-on main courses like the oven fried chicken wings we had last night. The ingredients are simple, the colors are lovely, and the only care you need take is to mix it 15-30 minutes ahead of time so the flavors can blend.
two conventional cucumbers*
one red onion
white wine vinegar
Peel and slice the cucumbers in fat slices or thick batonnets. My preference is to peel half the cuke (that is, run the peeler down the outside leaving strips of green skin in between) and then run a salad fork (held upside down) down the vertical length of the entire exterior to create 1/8" deep grooves--these will greatly improve the ability of the cucumbers to soak up dressing and onion flavor. And they look cool, too, but it's really for the other reason. Cut the onions in half in strips. Place all in a bowl, sprinkle over about 1 tblsp each oil and vinegar, add lots of salt and let sit. Adjust seasoning just before serving.
* Although most people think of cucumbers as a summer vegetable because that's when we grow them in the garden, for the gardenless there's no better time to buy cucumbers than now. They are smaller, crisper and sweeter in winter because they get all the water they need, where store-bought July cucumbers will often be dry and rubbery from too much sun and not enough water.