Place the zucchini and onion in a large, glass bowl and sprinkle the salt over top. Use your hands to evenly mix the salt throughout the zucchini. Cover and refrigerate overnight.
The following day, drain the zucchini in a colander and rinse well with cool water. Squeeze out excess water.
In a large pot or dutch oven over high heat, add zucchini, sugar, vinegar, cornstarch, nutmeg,, celery seed, and pepper. Stir to combine and bring to a boil, then reduce to simmer uncovered for 30 minutes.
Meanwhile, sterilize seven one-pint jars and lids to hold the relish. Pack the relish into the sterilized jars, making sure there are no spaces or air pockets. Fill the jars all the way to the top. Screw on the lids.
Place a rack in the bottom of a large stockpot and fill halfway with boiling water. Carefully lower the jars into the pot using a holder. Leave a 2-inch space between the jars. Pour in more boiling water, if necessary, until the tops of the jars are covered by 2 inches of water. Bring the water to a full boil, then cover and process for 30 minutes.
Remove the jars from the pot and place them on a cloth-covered or wooden surface, several inches apart, until cool. Once cool, press the top of each lid with a finger to ensure the seal is tight. The lid should not move up or down at all.