Combine the tomatoes, their juices, butter and onion 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, mashing any large tomato pieces with a spoon. Taste, and season with more salt if needed.

Toss the sauce with pasta, discarding the onion before tossing if you wish. Top with freshly ground black pepper and freshly grated Parmesan cheese, if desired. This recipe makes enough sauce for a pound of pasta.