On medium-high heat, in a small sauce pot, add in your vinegar, water, sugar, salt and chilli flakes. Heat until sugar and salt has dissolved then let cool.
Get a clean glass jar with a lid and add in your sliced red onions.
Once the brine has cooled, pour it into your glass jar with onions.
Store in the fridge for minimally 6 hours (the longer the better) and enjoy!