In a medium pot, whisk together the vinegar, water, salt and sugar. Bring to boil over high heat; then reduce to low. Add onions and bay leaf; cook for 1 minute, stirring occasionally. Remove from heat.
Add beet slices (or purple cabbage) and stir again. Let cool. Let marinate at least 30 minutes before serving. Serve on everything!!
Store onions in a glass container with tight-fitting lid in the refrigerator, like pickles, for 1 month (or more). They taste better the sooner you eat 'em though!