Place zucchini and onion slices in a large bowl. Add salt and then cover with 1 inch of water. Let stand 2 hours at room temperature and then drain thoroughly.
While the vegetables are soaking, prepare water bath canner and jars.
When you're vegetables are ready, start the brine.
In a stockpot, combine vinegar, sugar, mustard seed, celery seed and ground tumeric.
Bring to a boil and add drained zucchini and onions. Stir occasionally to ensure the sugar is dissolved completely.
Simmer mixture for 5 minutes.
Fill hot quart jars with vegetable mixture and pickling solution, leaving 1/2-inch headspace. Adjust lids and process according to the recommended time depending on your elevation:
Recommended process time for Pickled Bread and Butter Zucchini in a boiling-water canner.
Process Time at Altitudes of:
0 - 1,000 ft: 10 minutes
1,001 - 6,000 ft: 15 minutes
Above 6,000 ft: 20 minutes