Wash your jars, lids, screw bands, and canning tools in hot soapy water. Rinse thoroughly to remove all suds. Set aside to air dry on a clean kitchen towel.

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

Peel the onions and remove the root and stem ends. Cut the onions into 1/4-inch slices. Break apart to separate the segments. Set aside.

Add the vinegar, sugar, mustard seed, celery seed, turmeric, peppercorns, and salt to a medium saucepan. Bring to a boil over medium-high heat, stirring until the sugar and salt dissolve, then reduce the heat to medium.

Add the prepared onions to the brine, and simmer for 5 minutes, stirring occasionally, until the onions are heated through and slightly softened.

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

Run the bubble popper through the jar to release trapped air bubbles. Adjust the headspace again if needed. Wipe the rim to remove any residue, center a lid on the jar, and screw on a band until it is fingertip tight.

Use the jar lifter to place the filled jar back into the canner, and repeat with the remaining jars.

Once all the filled jars are in the canner, adjust the water level so it is 2 inches above the jar tops.

When processing time is complete, turn off the heat, remove the cover, and let the canner cool down and settle for about 5 minutes.

Spread a dry kitchen towel on the counter. Remove the jars from the canner and place them on the towel. Keep the jars upright, and do not tighten the bands or check the seals yet. Let the jars sit undisturbed for 12 to 24 hours.

After 12 to 24 hours, check to be sure the lids have sealed by pushing on the center of each lid. The lid should not pop up. If a lid flexes up and down, it did not seal. Refrigerate that jar and use it within a week.

Remove the screw bands and wash the jars. Label, date, and store in a cool, dark place, and use within 12 to 18 months for the best quality and flavor.

Let the pickled onions stand for several weeks before opening for the flavors to develop. Refrigerate the jar after opening.