Slice the zucchini and onions into pens. Put everything in a large saucepan, add salt and stir. Let stand and steep for 2 - 3 hours (also overnight).
Add sugar, herb vinegar and mustard and cook for about 20-30 minutes.
Put the spices through a sieve immediately after boiling, stir in firmly and bring to the boil once very briefly. Fill into glasses and enjoy either cold or warm.