Put either the prepared fresh tomatoes or the canned in a saucepan, add the butter, onion, and salt, and cook uncovered at a very slow, but steady simmer for 45 minutes, or until the fat floats free from the tomato. Stir from time to time, mashing any large piece of tomato in the pan with the back of a wooden spoon. Taste and correct for salt. Discard the onion before tossing the sauce with pasta. Serve with grated Parmesan.