In a large mixing bowl, stir the chopped zucchini, chopped onion and salt together. Let the mixture stand overnight. Transfer to a colander to drain. Rinse under cold water. Squeeze out the excess water and place the mixture in a large stock pot.
Stir in the vinegar, celery seed, black pepper, nutmeg, turmeric, cornstarch, and sugar. Bring the mixture to a very low boil, reduce heat and cook gently for 30 minutes. Add the chopped celery and chopped bell pepper. Cook for an additional 30 minutes.
Spoon the relish mixture into pint jars and process in a boiling water bath for 15 minutes. This recipe yields 7-1/2 pints of relish.