Thinly slice tomatoes and onions, place in a large bowl. (Note: You can use the slicing attachment on a food processor to speed this up, I usually don’t bother).

Toss vegetable mixture with pickling salt, cover with plastic wrap. Transfer bowl to the fridge to chill overnight.

The next day, drain excess liquid from the vegetables, using a large colander.

In a large pot over medium heat, combine vinegar and sugar. Add the drained vegetables, celery, and green peppers, stirring to combine.

Measure pickling spice onto a doubled-over piece of cheese cloth, gather edges and use a small piece of twine to secure into a little package; add to pot, along with turmeric and mustard powder.

Bring mixture to a rolling, full boil, lower heat to simmer. Simmer, stirring occasionally, until mixture is thick - 20 minutes.

While the chow chow mixture is simmering, Get a large pot of water boiling, or set up your canner per the manufacturer's instructions. I’ll usually use a large stock pot with a canner rack in it.

Remove spice packet, squeezing excess liquid back into the pot.

Use a sterilized canning funnel and sterilized ladle to ladle Chow Chow into sterilized canning jars, leaving about ⅛ inch head space. Use an air bubble tool to release any air bubbles, if you like,

Wipe off the top rims of the jars with a clean dampened cloth (I tend to use a clean, wet paper towel), top each with a new, sterilized lid, and carefully screw on a clean lid ring. I like to use a kitchen towel for this, the jars are HOT!

Carefully place your filled jars of relish into the boiling water pot, allow to process in this hot water bath for 20 minutes. (Add 5 minutes to the processing time for altitudes above 1,000 feet; add 10 minutes for altitudes over 6,000 feet.)

Note: There should be enough water to keep the jars fully submerged, the entire time.

CAREFULLY remove the jars, loosen the lids, and allow the hot jars to cool in a overnight.

The next day, check to make sure that all of the jars achieved a proper seal – try to push down in the middle of each lid. If it “pops”, it did not seal.

Any jars that didn’t seal after fully cooling to room temperature should be put in the fridge and used in the next few weeks.

Close caps all the way, and store the sealed mason jars in a cool dark place (ideally) for up to 1 year, chill well before eating.