Assemble all ingredients and tools so everything is within reach before beginning.

Combine the shredded zucchini and chopped onion in a large non-metal bowl. Sprinkle the canning salt over the mixture and work it in thoroughly with your hands, then cover and refrigerate overnight to draw out moisture.

The next day, transfer the zucchini and onion to a colander and rinse well under cool running water. Press or squeeze the mixture to remove as much liquid as you can, then set the drained zucchini aside.

In a large pot, combine the sugar, vinegar, chopped red and green bell peppers, and cornstarch. Stir in the celery seed, nutmeg, turmeric, and black pepper until evenly distributed and the cornstarch is dissolved.

Add the drained zucchini to the pot and bring the mixture to a boil over medium-high heat. Reduce the heat to medium-low and simmer, stirring occasionally, for about 30 minutes or until the relish has thickened and the peppers are tender.

While the relish simmers, sterilize seven 1-pint jars and their lids by boiling them or using a hot dishwasher cycle; keep the jars hot until you are ready to fill to prevent breakage.

Spoon the hot relish into the sterilized jars, pressing down to remove any air pockets and filling each jar to the top. Wipe the rims clean and secure the lids snugly.

Place a rack in the bottom of a large stockpot and bring water to a boil. Using a jar lifter, carefully lower the filled jars onto the rack, leaving about 2 inches of space between them and ensuring the jars are covered by at least 2 inches of water. Bring the water back to a full boil, cover, and process the jars for 30 minutes.

Remove the jars from the pot and set them several inches apart on a cloth-covered or wooden surface to cool. Once cool, press the center of each lid to verify a proper seal—the lid should not move up and down.