Peel the red onion and slice it very thinly (a mandoline works best for uniform slices).
In a small saucepan, combine apple cider vinegar, water, sugar, and salt. Add any optional flavorings.
Place the sliced onions in a clean glass jar or container. Pour the brine over the onions, making sure they are fully submerged.
The onions are ready to eat in at least 30 minutes, but flavour improves after a few hours or overnight.