Place green tomatoes, green bell peppers, red bell peppers, and onions in a food processor; pulse until coarsely chopped. Transfer to a large bowl. Pour boiling water over vegetable mixture; drain. Transfer vegetable mixture to a large pot. Add vinegar, sugar, salt, turmeric, and mustard; stir to combine. Bring mixture to a boil until vegetables are softened, 10 minutes.
Sterilize the jars, lids, and rings in boiling water for at least 5 minutes. Pack the relish into the hot, sterilized jars, filling the jars to within 1/4 inch of the top. Run a knife or a thin spatula around the insides of the jars after they have been filled to remove any air bubbles. Wipe the rims of the jars with a moist paper towel to remove any food residue. Top with lids, and screw on rings.
Place a rack in the bottom of a large stockpot and fill halfway with water. Bring to a boil and lower jars into the boiling water using a holder. Leave a 2-inch space between the jars. Pour in more boiling water if necessary to bring the water level to at least 1 inch above the tops of the jars. Bring the water to a rolling boil, cover the pot, and process for 15 minutes.
Remove the jars from the stockpot and place onto a cloth-covered or wood surface, several inches apart, until cool. Once cool, press the top of each lid with a finger, ensuring that the seal is tight (lid does not move up or down at all). Store in a cool, dark area.
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