Finely slice the red onion, you can do this with a knife if you have good knife skills or use a mandolin. In a clean jam jar place the sugar, salt and vinegar and stir. Add the sliced onion to the jam jar. Top up with freshly boiled water from the kettle to just cover the onions. Screw the lid on and give it a little shake to distribute the ingredients. Place in the fridge for a couple of hours before using.