Add all ingredients except the onions to a medium sauce pan and stir until the sugar is dissolved. Heat the mixture on high.
Add the sliced onions and bring to a boil. Reduce to a simmer and cook for 20 minutes.
Store in the refrigerator. Keeps up to 2 months.
Helpful Tip: Have canning equipment? Make a large batch and can them for long storage. Take a jar out when you need it!