Place the onions in a medium saucepan and pour in enough water to cover. Bring to a boil, and remove from the heat. Strain and set the onions aside. Combine all the remaining ingredients in the saucepan. Bring to a boil, reduce to a simmer an additional 10 minutes. Transfer the mixture to a container, cover and refrigerate at least a day before serving. Makes 5 1/2 cups Pickled onions will keep in the refrigerator up to a month. SOURCE: Mesa Mexican by Mary Sue Milliken and Susan Feniger with Helena Siegel.