In a food processor, chop tomatoes, onions, and peppers in batches, using the pulse mechanism. Stir together in a large stock pot. Sprinkle with the salt and refrigerate overnight.
Drain off liquid and stir in vinegar, mustard and sugar. Bring to a slow boil; continue boiling until tender (about 15 minutes).
Pack into canning jars and refrigerate, or process according to safe canning methods. Makes about 10 pints.