Place red onion slices into a 16 ounce (pint) jar. If you’re adding seasonings, add them to the jar now.
Combine apple cider vinegar, water, sugar, and salt. Bring to a boil. Remove the brine from the heat and let cool for a few minutes.
Carefully pour brine into jar, covering the onion slices.
Let cool at room temperature for about an hour. Place a lid or cover on the jar and refrigerate for two days before enjoying. Store the red onions in the refrigerator. They keep for about 2 weeks.