Bring medium pot with water to a boil. Add onions to the pot and boil for 2-3 minutes.
Take onions out of boiling water and drain. While onions are still hot, transfer onions to a medium size bowl and pour apple cider vinegar. Let the onions soak in the vinegar and let it cool.
Once onions have cooled, transfer onions and vinegar into sanitized, air tight jars. Keep jars refrigerated.