Use food processor or grater to shred zucchini and onions. Put in large bowl.
Add salt to mixture.
Cover bowl and put in fridge to sit overnight.
After sitting overnight put zucchini in a strainer and rinse well with cold water.
Use spoon to press out as much moisture as possible from zucchini.
Pour mixture into a large, heavy pot.
Grind peppers and add to mixture.
Add sugar, celery seed, vinegar, nutmeg, dry mustard, turmeric, cornstarch, and black pepper. Stir well.
Turn stove to low and cook relish for 30-60 minutes. The more moisture the zucchini has, the longer it will need to cook. It is OK for it to be a little runny and watery when bottling.
Pour mixture into hot pint sized mason jars. Process in hot water bath, using safe canning procedures, for 15 minutes.