Wash tomatoes. Remove peels, stem ends, and cores. Chop tomatoes. In a 6- or 8-quart Dutch oven or kettle combine the chopped tomatoes, onions, vinegar, sugar, chili powder, salt, mustard, and red pepper. Bring to boiling; reduce heat. Boil gently, uncovered, about 2 hours or until mixture is about the thickness of catsup, stirring occasionally.
Immediately ladle chili sauce into hot, clean half-pint jars, leaving 1/2-inch headspace. Wipe jar rims and adjust lids. Process in a boiling-water canner for 15 minutes (start timing when water returns to boil). Remove jars from canner; cool on racks.