If using fresh tomatoes peel them and coarsely chop them. If using canned, coarsely chop them. Trim both ends of the onion, peel it and cut it half lengthwise.
Put the tomatoes, butter, onion, and salt in a 4-to-5-quart saucepan and place over medium heat. When the tomatoes begin to bubble, lower the heat to a slow but steady simmer. Cook, stirring every 10 to 15 minutes until the tomatoes are no longer watery and the sauce has reduced, about 45 minutes, depending on the size and shape of the pot.
When the sauce is done the butter will have separated from the tomatoes and there should not be any liquid left. When you toss the pasta with the sauce, add freshly grated Parmigiano-Reggiano.
Yield Makes enough for 1 pound of dried pasta