Amy LeBlanc credits this recipe to her best friend, Janet Bedard, whose family has handed it down through a couple of generations. This relish repeatedly wins blue ribbons whenever it is entered at the fair. Amy is an organic gardener and inspector who has just gone into business selling her preserves and vinegars.
In a large glass or stainless steel bowl, combine the zucchini, onion, red and green peppers, tomatoes, and celery. Add the salt and stir well. Place a plate right on the vegetables and press down (a stack of smaller bowls works well). Let stand overnight.
In the morning, drain the vegetables and rinse well 2 or 3 times until most of the salty taste is gone. Transfer to a large stainless steel pot and add the vinegar, sugar, black pepper, mustard, turmeric, allspice, celery seed, and cornstarch-water mixture. Stir well. Simmer for 45 minutes, or until the sauce is clear. Fill ½-pint or 1-pint jars with the relish, cover, and process in a boiling-water bath for 10 minutes for ½-pint jars or 15 minutes for 1-pint jars.