Toss both cabbages with the teaspoon of salt in a colander and leave over the sink for 20 minutes.
Squeeze the cabbage gently with your hands to release the liquid, then spread it on a clean tea towel and pat dry. Don’t rinse it — you want the seasoning.
Whisk the dressing ingredients together in a large bowl until smooth.
Add the cabbage, carrot, spring onion and parsley, and toss thoroughly until everything is lightly coated.
Taste and adjust — it usually wants more pepper and sometimes another splash of vinegar.
Serve within the hour. It will hold up for three or four hours if it has to.