Wash zucchini. Slice off the stem and blossom ends. Cut zucchini crosswise into 1/4-inch-thick slices. Measure 12 cups zucchini slices.
In an extra-large nonmetal bowl, combine the 12 cups zucchini and the 1 cup onion slices. Sprinkle with salt; toss gently to coat. Top with 2 inches of crushed ice. Weight down mixture with a heavy plate. Allow to stand at room temperature for 2 hours.
Remove any remaining ice from zucchini mixture; transfer zucchini mixture to a colander set in a clean sink and drain.
In a 5- to 6-quart stainless-steel, enamel, or nonstick heavy pot, combine vinegar, sugar, mustard seeds, celery seeds, peppercorns, and turmeric. Bring to boiling, stirring until sugar dissolves. Add zucchini mixture. Return to boiling, stirring frequently; reduce heat. Simmer, uncovered, for 5 minutes.
Ladle hot mixture into hot, sterilized pint jars, leaving a 1/2-inch headspace. Wipe jar rims; adjust lids.