Tomato Sauce with Onion and Butter

Put the prepared tomatoes in a saucepan with the butter, the onion halves, and salt.

Simmer uncovered at a very slow, steady simmer for 45 minutes, until the fat floats free from the tomato. Stir from time to time, pressing down on larger pieces of tomato with the back of a wooden spoon.

Taste and correct for salt. Discard the onion before using the sauce with pasta.