Ingredients for 4 servings:
1. 2 cups canned plum tomatoes (whole, peeled, chopped, with their juices about one 28-oz. can)
2. 5 tablespoons unsalted butter
3. 1 medium yellow onion, peeled and cut in half
4. salt, to taste
1. Combine the tomatoes, their juices, the butter, and the onion halves in a medium saucepan.
2. Add a pinch or two of salt. Place over medium heat and bring to a simmer. Cook, uncovered, at a very slow but steady simmer, adjusting the heat as necessary, for about 45 minutes, or until droplets of fat float free from the tomato.
3. Stir occasionally, mashing any large pieces of tomato with the back of a wooden spoon. Taste and salt as needed.
4. Discard the onion.