Prepare cucumbers by peeling and slicing them very thin. Thinly slice the onions. Add them to the cucumbers and sprinkle on the salt and sugar, and the vinegar.

Mix well and let rest in the refrigerator for about 20 minutes before serving.

You may want to drain off any excessive juices that accumulate before serving as well, depending on how long they sit before serving. If making the night before, just store in the refrigerator and then drain before serving (I happen to like the excess liquid it makes).

The salad will be fine in the refrigerator for about one week.