Combine all ingredients except the onions in a heavy medium saucepan and set it over medium-high heat. Bring the mixture to a boil, then add the onion slices, separating them into individual rings. Let the mixture come back to a boil, then reduce the heat and cook until onions soften and wilt, 3 to 4 minutes.
Remove the saucepan from the heat, cover, and let the mixture cool completely. Transfer the onions and their pickling liquid to a quart-size glass jar or divide among Tupperware. Cover and refrigerate for up to 2 weeks in the refrigerator.