Gently scrub the summer squash and rinse well under clean running water. Trim off the stem and blossom ends, cut into 1/4-inch slices, and add them to a large prep bowl.

Peel and cut the onion into 1/4-inch slices, and add them to the prep bowl.

Sprinkle the salt over the vegetables, then cover with cold ice water. Stir to combine, then let the vegetables soak in the salted water at room temperature for at least 3 hours.

Place the water bath canner on the stove, add the jar rack, place jars upright on the rack, and add water to cover. Bring the canner to a simmer (180˚F) for at least 10 minutes, and keep warm.

After soaking, drain the vegetables, discard the water, and set the vegetables aside. (Optional: You can rinse the vegetables to remove excess salt if desired).

Combine the vinegar, sugar, mustard seeds, celery seeds, and turmeric in a large saucepan and bring it to a boil over medium-high heat while stirring until the sugar dissolves. Reduce the heat to a simmer, and keep warm as you fill your jars.

Spread a dry towel on the counter. Use the jar lifter to remove a warm jar from the canner. Drain the water back into the canner and place it on the towel. Keep the remaining jars in the canner so they stay warm.

Tightly pack the squash and onion mixture into a warm jar, then ladle hot pickling brine to cover, leaving a 1/2-inch headspace. Run the bubble popper through the jar to release air bubbles, and adjust the headspace again if needed.

Wipe the rim clean with a damp paper towel. Center a lid on the jar, and screw on the band until it is fingertip tight. Use the jar lifter to place the jar back into the canner, and repeat with the remaining jars.

Once the filled jars are all in the canner, adjust the water level to two inches above the jar tops.

Cover the canner and bring it to a rolling boil over medium-high heat. Once it boils, set a timer and process both pints and half-pints in a boiling water bath canner for 20 minutes at altitudes of less than 1,000 ft. Adjust processing time for your altitude if necessary. (See notes)

When the processing time is complete, turn off the heat, remove the cover by tilting the lid away from you so that steam does not burn your face, and allow the canner to cool down and settle for 5 minutes.

Spread a dry kitchen towel on the counter. Use a jar lifter to remove the jars one at a time from the canner. Keep the jars upright and place them on the towel. Do not tighten ring bands or test the seals yet. Let the jars sit undisturbed for 12 to 24 hours to cool.

After the jars have cooled for at least 12 hours, check to be sure the lids have sealed by pushing on the center of the lid. The lid should not pop up. If the lid flexes up and down, it did not seal. Place the jar in the refrigerator, and use within a month.

Remove the ring bands, wash, label, date the jars, and store in a cool, dark place between 50 to 70 degrees F for 12 to 18 months. Let the pickles sit for 4 to 6 weeks to develop flavor. Once the jar is open, refrigerate and use it within a month. Yields about 5 pint jars or 10 half pints jars.