In a medium bowl, combine the apple cider vinegar, water, sugar, and sea salt. Stir well until the sugar and salt are fully dissolved. This helps create a smooth and balanced pickling liquid.

Place the thinly sliced red onion into a clean glass jar, such as a mason jar. Spread the slices out so they are not tightly packed.

Slowly pour the vinegar mixture over the onions. Make sure all the slices are completely covered with liquid. If needed, gently press them down with a spoon.

Leave the jar at room temperature for about 1 hour. During this time, the onions will soften and absorb the flavor.

After one hour, cover the jar with a lid and transfer it to the refrigerator. Store the onions in the pickling liquid for up to two weeks.

When ready to use, remove the onions with a fork or slotted spoon to avoid adding too much liquid to your dish.