Dice or chop peeled onions in a food processor, and combine with shredded zucchini. Add salt, stir, and set-aside overnight.
Rinse with cold water and drain well.
Combine sugar and cornstarch, add to the vinegar along with the dry mustard, nutmeg, turmeric, celery seed, and black pepper in a non-reactive pot. Add the zucchini mixture and the peppers and cook slowly for 30 minutes.
Pack relish into prepared jars leaving 1/4 inch headspace. Using a bubble remover or the handle of a wooden spoon, remove any air bubbles. Wipe the rims, adjust the two-piece caps and process in a boiling-water bath for 10 minutes.
When processing is complete, turn off the heat and remove the canner lid. Let jars sit in canner for 5 minutes to acclimate to the temperature change before removing.
Using a jar lifter, remove the jars from the canner and place on a towel. Leave the jars undisturbed for 12-24 hours. Check the jars to make sure they all sealed, wipe the tops, label, and move to pantry or food shelves. If a lid did not seal within 24 hours, either reprocess or refrigerate right away.