Pass zucchini, onions, and bell pepper through a meat grinder into a large bowl; season with salt. Refrigerate at least 10 hours to overnight.

Combine zucchini mixture, sugar, vinegar, celery seeds, and turmeric in a large pot; bring to a boil, and cook, stirring constantly, 15 to 20 minutes. Stir in cornstarch; boil until thickened, about 5 minutes more.

Inspect 12 half-pint jars for cracks and rings for rust, discarding any defective ones. Immerse in simmering water until relish is ready. Wash new, unused lids and rings in warm soapy water.

Pack relish into hot, sterilized jars, filling to within 1/4 inch of tops. Run a clean knife or thin spatula around insides of jars to remove any air bubbles. Wipe rims with a moist paper towel to remove any residue. Top with lids and screw rings on tightly.

Place a rack in bottom of a large stockpot and fill halfway with water. Bring to a boil and lower jars 2 inches apart into the boiling water using a holder. Pour in more boiling water to cover jars by at least 1 inch. Bring to a rolling boil, cover, and process for 10 minutes.

Remove jars from the stockpot; rest, several inches apart, until cool. Press centers of lids with a finger to ensure lids do not move up or down. Remove rings for storage and store in a cool, dark area.