Combine all ingredients except onions in a medium pot. Bring to a boil.
When mixture comes to a boil, add onions. Stir and boil for 1 minute. Reduce heat and simmer for 8 to 10 minutes. You want to cook the onions down some, but you don't want them to get totally soft. They should still have a little crunch.
Refrigerate until ready to serve. Will last several weeks in the refrigerator.