Drain the beets,canned or fresh, reserving the liquid from can or 1/2 cup of the cooking water. Combine the vinegar and the water in the bowl, then add the sugar and stir to dissolve. In a small bowl, combine beets, vinegar mixture,sliced onion and just enough of the reserved beet liquid to cover the beets.
Refrigerate, tightly covered, for several hours or overnight. Serve chilled or at room temperature. The will keep in refrigerator for a week.