Cut the onion. Using a sharp knife, cut the onion into very thin slices.
Make the pickling juice. Heat the apple cider vinegar, brown sugar, and salt in a medium saucepan over medium heat. Cook until all ingredients have dissolved. Remove from heat and cool.
Add the onions. Add the sliced onions to a wide-mouth mason jar. Pour the pickling juice over the onions until they are fully submerged in the liquid.
Pickle the onions. Tightly screw a lid on the jar. Refrigerate for at least 2 hours before serving.
Use and store. Onions can be stored in a sealed jar in the refrigerator for up to 1 week.
