Place the vinegar, cane sugar, sea salt, and 1 cup of water in a small saucepan and bring to a boil. Add the sliced onions and simmer for 15 minutes. Turn off the heat and let it cool completely. Once cool, place the onions and all the liquid in a glass container with a lid, and refrigerate for up to 3 weeks. When you are ready to use, scoop some onion out and add it to your favorite breakfast burrito or bowl of pinto beans and rice.