Step 1: Combine the chopped tomatoes, cabbage, peppers, and onions in a large glass or plastic bowl. Sprinkle with canning salt and toss well to coat.
Step 2: Cover the bowl and let the vegetables sit at room temperature for at least 8 hours or overnight to draw out moisture.
Step 3: Drain the vegetables in a colander and rinse thoroughly with cold water to remove excess salt. Press down on the vegetables to squeeze out as much liquid as possible.
Step 4: In a large stainless steel stockpot, combine the apple cider vinegar, sugar, mustard seeds, celery seeds, turmeric, and red pepper flakes. Bring the mixture to a boil over medium-high heat, stirring until the sugar is dissolved.
Step 5: Add the drained vegetables to the boiling brine. Reduce the heat and simmer for 20 minutes, stirring occasionally, until the vegetables are tender but still retain some crunch.
Step 6: Ladle the hot chow chow into sterilized pint jars, leaving 1/2 inch of headspace. Remove air bubbles, wipe rims, and adjust lids.
Step 7: Process the jars in a boiling water bath canner for 10 minutes. Remove and let cool undisturbed for 24 hours before checking seals and storing.