Bring a large pot of water to a boil and add your container. Boil for 5 minutes to sterilise it, remove and leave to cool
Julienne your onion and separate the slices by hand. Add to the container.
Add the salt and sugar. Massage it by hand and ensure the salt and sugar is evenly distributed among the onions.
Leave to rest for 5 minutes. By this point, your onions should have sweated out a lot of liquid and started to soften.
Add in the vinegar and leave for another 5 minutes.
Add the boiling water, mix and seal the pot. Leave aside for another 30 minutes and watch the colour intensify! These will now be ready to eat straight away and can last up to a couple of months in the fridge.