Pack the sliced red onion into a large jar with a tight fitting lid.
Whisk together the hot water, apple cider vinegar, sugar and salt until well combined and sugar and salt have dissolved.
Pour the liquid over the sliced onions in the jar, making sure the onions are completely submerged. Set the jar aside to cool for 30 minutes. Use immediately, or cover the jar and store in the fridge for up to 4 weeks.