Wash the jars, lids, screw bands, and canning tools in hot soapy water. Rinse well to remove all suds.

Place the jar rack into the water bath canner, place jars in the canner, and add water to cover. Bring the canner to a simmer (180˚F) for 10 minutes, and keep the jars warm until you are ready to fill them.

Rinse the squash well under clean running water. Remove the blossom and stem ends. If the squash is large, you may want to remove the seeds as well. Finely chop or shred the squash using a food processor. Add to a prep bowl.

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.

Add the prepared summer squash and onions and stir to combine. Return to a boil, and continue to boil for 5 minutes, then reduce the heat and keep warm as you fill your jars.

Spread a 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.

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 15 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 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. Once the jar is open, refrigerate and use it within a month. Yields about 5 pints or 10 half pints.