Step 1: Thoroughly wash and finely chop the green tomatoes, cabbage, and onions. You can use a food processor for this to save time, ensuring the pieces are uniformly small.
Step 2: In a large, non-reactive bowl, combine the chopped vegetables and sprinkle evenly with 1/4 cup of canning salt. Toss well to coat. Cover and let sit at room temperature for at least 4 hours, or overnight, to draw out excess moisture.
Step 3: Drain the vegetables thoroughly in a colander. Rinse them several times under cold running water to remove excess salt. Press out as much remaining water as possible from the vegetables; this is crucial for a crisp chow chow.
Step 4: In a large stockpot or Dutch oven, combine the white vinegar, granulated sugar, mustard seeds, celery seeds, ground turmeric, and optional red pepper flakes. Bring to a boil over medium-high heat, stirring until the sugar is dissolved.
Step 5: Add the well-drained vegetables to the simmering brine. Bring the mixture back to a gentle boil, then reduce heat to a simmer. Cook for 20-30 minutes, stirring occasionally, until the vegetables are tender-crisp and the relish has thickened slightly.
Step 6: If canning, prepare your canning jars and lids by sterilizing them. Carefully ladle the hot chow chow into sterilized pint or quart jars, leaving 1/2 inch of headspace. Remove any air bubbles and wipe jar rims clean.
Step 7: Place lids and screw on bands finger-tight. Process the filled jars in a boiling water bath canner for 15 minutes (adjusting for altitude if necessary).
Step 8: Turn off the heat, remove the canner lid, and let the jars sit in the hot water for 5 minutes before carefully removing them to a wire rack to cool completely for 12-24 hours. Check seals: lids should be concave and not flex. Store properly sealed jars in a cool, dark place for up to 1 year. Refrigerate unsealed jars and consume within 2-3 weeks.