Combine the tomatoes, butter and the onion halves in a saucepan. Add a pinch or two of salt.

Place over medium heat and bring to a simmer. Cook, uncovered, for about 45 minutes. Stir occasionally, keeping onion pieces with the cut sides down in the sauce as much as possible.

Discard the onion before tossing the sauce with pasta. This recipe makes enough sauce for a pound of pasta.