My grandmother use to make this for the hay crews at the noon dinnner. It is simple and so refreshing on a hot day.
Take a cucumber, peeled or unpeeled, and slice into rings. Thinly slice a small onion. Put these ingredients into a bowl and cover with cold water. Add vinegar to taste. Add black pepper to taste and dash of salt if you wish. Refrigerate for a few hours (or overnight) before serving.