Use a food processor to finely chop green tomatoes, cabbage, green and red sweet peppers, and onion, processing a portion at a time, using several on/off turns of the processor. (Or, finely chop by hand.) Place vegetables in a large bowl. Sprinkle with the 2 tablespoons pickling salt; stir well. Cover and chill overnight. Rinse well in colander under running water; drain.

In a large pot stir together sugar, mustard seed, celery seed, and turmeric. Stir in vinegar and water. Bring to boiling, stirring to dissolve sugar. Stir in vegetables. Return mixture to boiling, stirring frequently. Remove from heat. Ladle hot relish into hot, sterilized half-pint or pint canning jars, leaving a 1/2-inch head-space. Wipe jar rims and adjust lids. Process in boiling water canner for 5 minutes for half-pints or 10 minutes for pints (start timing when water begins to boil). Store sealed jars up to 3 months. Makes 6 to 7 half-pints or 3 to 4 pints (eighty-four 1-tablespoon servings).