The point of this recipe is to make a large amount at one time, when tomatoes are in season. If you’re canning it, stick closely to the recipe; adding additional fresh vegetables will change the pH so it’s unsafe for water-bath canning. If you’re freezing it, then it’s fine to throw in peppers, mushrooms, fresh garlic, whatever you want. This recipe makes 6-7 quarts – you can use a combination of pint and quart canning jars or freezer boxes.
Soften onions in a heavy 3-gallon kettle – add a small amount of water if necessary but no oil if you are canning (very important!).
Add pureed tomatoes and all seasonings, bring to a boil, and simmer on low heat for two to three hours until sauce has thickened to your liking. Stir frequently, especially toward the end, to avoid burning.
Meanwhile, heat water in canner bath, sterilize jars in boiling water or dishwasher, and pour boiling water over jar lids.
Add 2 tbsp of lemon juice OR ½ tsp. citric acid to each quart jar, (half that much to pint jars). This is ensures that the sauce will be safely acidic.
When the sauce is ready, ladle it into the jars leaving ½ inch headspace. Put jars into canner and boil for 35 minutes. Remove, cool, check all seals, label and store for winter.