2 large red onion, thinly sliced
1 cup apple cider vinegar
1/2 cup water
1 teaspoon sea salt
Place the onion slices, apple cider vinegar, water, and sea salt in a large mason or canning jar. Tightly cover lid and shake jar until well combined. Open the lid and push the onions down to submerge in the liquid. Tighten lid and leave on the counter for 1 hour. Use immediately or store in the fridge for 1-2 weeks.