Cover zucchini and onion slices with 1 inch of water and the salt. Let stand 2 hours, then drain thoroughly.
Combine vinegar, sugar, mustard seed, celery seed, and turmeric. Bring to a boil.
Add zucchini and onions and simmer 5 minutes.
Fill hot jars with mixture and pickling solution, leaving 1/2-inch headspace. Remove air bubbles, adjust headspace, wipe rims, and apply lids.
Process pints or quarts 10 minutes at 0–1,000 ft, 15 minutes at 1,001–6,000 ft, or 20 minutes above 6,000 ft. The tested low-temperature pasteurization alternative may also be used when following NCHFP directions exactly.