Combine the tomatoes and juice in a saucepan, add the butter, onion, and a pinch of salt.
Place over medium heat and bring to a simmer. Cook, uncovered, for about 45 minutes. Stir occasionally, mashing any large pieces of tomato with a spoon. Add salt as needed.
Discard the onion before tossing the sauce with pasta. Serve with freshly grated Parmigiano-Reggiano cheese for the table.