In a large non-reactive bowl, such as one made of glass or ceramic, combine the finely chopped green tomatoes, onion, and bell pepper. Sprinkle the kosher salt over the vegetables and stir thoroughly to coat. Cover the bowl and let the mixture stand at room temperature for at least 4 hours or up to overnight.
Drain the vegetable mixture in a large colander, rinsing it lightly under cold water to remove the excess salt. Use your hands to gently squeeze out any remaining liquid from the vegetables.
In a large, heavy-bottomed pot, such as a Dutch oven, combine the apple cider vinegar, granulated sugar, mustard seeds, celery seeds, and red pepper flakes. Bring the mixture to a boil over medium-high heat, stirring frequently until the sugar is completely dissolved.
Add the drained vegetable mixture to the pot and return the liquid to a boil. Reduce the heat to a gentle simmer and cook, uncovered, for 20 to 25 minutes. Stir occasionally until the vegetables have softened but still retain a slight crunch and the liquid has thickened slightly.
While the chow chow is cooking, prepare a water bath canner and sterilize two pint-sized canning jars according to manufacturer instructions, or have clean containers ready for refrigeration.
Carefully ladle the hot chow chow into the prepared jars, leaving 1/2 inch of headspace. Wipe the rims clean, apply the lids and bands, and process in the water bath canner for 10 minutes if preserving. For immediate use, simply let the chow chow cool completely before covering and refrigerating.