Cut the cabbage into quarters and discard the outer leaves. Cut away the hard central core and shred the remaining cabbage finely. Layer the cabbage in a bowl with salt then cover and leave to stand overnight.
In a saucepan gently heat the vinegar and sugar until the sugar has dissolved. Set aside to cool.
The following day thoroughly rinse the cabbage and pat dry with kitchen roll.
Pack into a sterilised Kilner® Jar adding a slice or two of onion as you go.
Pour in the vinegar until the cabbage is covered then put on the lid. Leave for a week before eating.