Pickled Bread and Butter Zucchini

16-cups fresh zucchini, sliced
4-cups onions, thinly sliced
½-cup canning or pickling salt
4-cups white vinegar (5%)
2-cups sugar
4-tbsp mustard seed
2-tbsp celery seed
2-tsp ground turmeric

Cover zucchini and onion slices with 1 inch of water and salt
Let stand 2 hours and drain thoroughly
Combine vinegar, sugar, and spices
Bring to a boil and add zucchini and onions
Simmer 5 minutes and fill jars with mixture and pickling solution, leaving 1/2-inch headspace
Adjust lids and process