Place the sliced red onion into a clean pint-sized mason jar or heatproof container.

In a small saucepan, combine the water, apple cider vinegar, sugar, and salt. Bring to a gentle boil, stirring until the sugar and salt dissolve. Remove from the heat and let cool for 1–2 minutes. Stir in the red pepper flakes, if using.

Carefully pour the warm pickling liquid over the onions, making sure they are fully submerged. Let cool to room temperature, about 30 minutes.

Seal the jar and refrigerate. The onions are ready to enjoy after cooling and will continue to develop flavor over the next 24 hours. Store in the refrigerator for up to 1 month. Enjoy!