Place tomatoes in an 8- or 10-quart Dutch oven or kettle. Heat to boiling, stirring occasionally. Reduce heat to medium. Cook, uncovered, for 20 minutes. Press tomatoes through a food mill; return tomatoes to Dutch oven. Discard seeds and pulp.
In a medium saucepan combine water, onion, sweet pepper, and, if desired, mushrooms. Cook over medium heat, stirring often, until onion and pepper are soft. Add to tomato mixture. Stir in garlic, parsley, brown sugar, fennel seed (if desired), salt, basil, oregano, marjoram, and pepper. Bring to boiling; reduce heat. Simmer, uncovered, about 2 hours or until reduced by half, stirring frequently. (Measure the depth with a ruler at the start. At the finish, the depth should be half of the original measure.)
Ladle hot sauce into hot, clean quart or pint canning jars, leaving 1-inch headspace. Wipe jar rims; adjust lids. Process jars in a pressure canner at 10 pounds pressure for 25 minutes for quarts or 20 minutes for pints. Allow the pressure to come down naturally. Remove the jars from canner; cool on racks. Makes 4 pints (16 one-half cup servings).