Combine cabbage, green tomatoes, onion, bell pepper, and jalapeno in a large bowl. Add 2 tablespoons salt and toss well. Cover and refrigerate at least 4 hours, or overnight. This step pulls moisture out of the vegetables so the finished relish isn't watery.
Drain the salted vegetables in a colander, then rinse well under cool water to wash off excess salt. Drain again and press gently to squeeze out moisture. Set aside.
In a saucepan or pot, combine the vinegar, brown sugar, packed, celery seed, dry mustard, ground turmeric, ground ginger, ground allspice, and crushed red pepper flakes. Bring to a boil, then lower the heat and simmer about 3 minutes until the sugar fully dissolves.
Add the drained vegetables to the simmering brine. Return to a gentle boil, then simmer about 10 minutes, until the cabbage softens but still has a little bite. Taste and adjust the heat here - this is where you decide how spicy the batch turns out.
Pack the warm chow chow and the liquid into a clean quart jar. Let it cool, then refrigerate. It's good after a few hours but best after a day or two once the flavors settle. Keeps about 2 weeks in the fridge.