Place sliced onions in a medium saucepan and cover with water. Bring to a boil for one minute. Drain.

Return the onions to the pan and add cider vinegar, salt, and just enough cold water to barely cover the onions. Bring to a boil over high heat and cook for one minute.

Transfer the onions and brine to a glass jars. Let cool, then cover and store in the refrigerator for up to 3 weeks. Serve with burgers, hot dogs, salads, tacos and sandwiches.