Prepare a hot water bath canner, jars and two-piece lids for canning (optional, if canning).

Cover the zucchini and onion slices with 1 inch of water and the pickling or canning salt. Let stand 2 hours before draining thoroughly.

Combine the vinegar, sugar, mustard seed, celery seed, and ground turmeric in a large saucepan.

Bring to a boil, add the sliced zucchini and onion, and simmer for 5 minutes.

Working quickly, fill the prepared, hot jars with the vegetable mixture and pickling solution, leaving 1/2-inch headspace.

Remove air bubbles using a wooden chopstick or plastic utensil and adjust headspace if needed.

Adjust lids to fingertip tight and process for 10 minutes (see notes below if canning at higher altitudes).

When the timer goes off, turn off the heat and let the processed jars sit in the hot water for 5 minutes before removing and transferring to a clean surface.

Give the canned zucchini pickles 12 to 24 hours to come to room temperature.

Check the seals on each jar (they should be slightly concave), remove the rings (save for the next use), and store in a cool, dry place. If any of the jars haven't sealed properly, store in the fridge and enjoy within a couple of weeks.