Coarsely grind zucchini and onion with food grinder or food processor. Mix zucchini, onion, and salt in a large bowl. Cover and refrigerate overnight.
The next day, rinse zucchini and onion with cold water, strain into a sieve, discard water, return to bowl, rinse again.
In a large pot, add vinegar, sugar, pepper, celery salt, nutmeg, dry mustard, turmeric, and cornstarch and bring to a boil. Add zucchini and onion and bring back to a boil. Reduce heat and let simmer uncovered for 30 minutes.
Prep jars for canning if storing, or just add to jars and let cool before storing in fridge.