Combine the tomatoes, their juices, the butter and the onion halves in a saucepan. (Add a pinch or two of salt, if using.)

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. This recipe makes enough sauce for a pound of pasta.