In a saucepan over medium heat, bring vinegar, water, sugar, and salt to a boil. Stir until sugar and the salt has dissolve.
Place sliced onions into a large, clean, jar. Add a bay leaf, and peppercorns to jar.
Pour the vinegar mixture over the onions.
Allow to cool, then seal with lid and keep refrigerated until ready to use. Can keep up to 2 week, covered, in the refrigerator.
Notes
If you prefer, you can omit the bay leaves and/or peppercorns.It's best to allow the pickled onions to "marinate" for at least 30 minutes before serving.