Put enough scrubbed and trimmed zucchini (about 16) through the coarse blade of a food grinder to measure 12 cups. Grind enough onion to measure 4 cups. In a large ceramic or glass bowl, combine the zucchini, onion, and 5 tablespoons pickling salt (or table salt), cover, and chill the mixture overnight. Drain the zucchini mixture in a colander, rinse several times in cold water, and drain thoroughly.

In a kettle, combine 5 cups sugar, 3 cups cider vinegar, 1 tablespoon dry mustard, 1 1/2 teaspoons celery seed, 1 teaspoon pepper, and 3/4 teaspoon turmeric, and bring the liquid to a boil, stirring until the sugar is dissolved. Add the zucchini mixture, 1 red sweet pepper, finely chopped, 1 green pepper, finely chopped, and simmer the mixture, stirring frequently, for 30 minutes. Ladle the relish into 5 hot sterilized pint jars, leaving 1/8 inch headspace. Whip the rims of the jars and seal immediately. Put jars in a hot water bath for 15 minutes.