Slice the zucchini and onion into thin strips, cover with the salt and leave to stand in a bowl overnight. Drain the onion and zucchini mixture (the juice is NOT used). Cut the peppers into thin strips and add. Put in a large saucepan.
Add the vinegar, sugar, mustard, turmeric, nutmeg, celery salt, pepper and the ketchup to the zucchini mixture, mix everything together well. Let simmer for about 1/4 hour. Pour hot into screw-top jars and close.
Makes approx. 8 glasses with 360g content.
The sauce tastes good with grilled, Viennese dishes or serves as a sauce for schnitzels.