1 28-ounce can of plum tomatoes (preferably San Marzano- you can read about them here.)
5 tablespoons unsalted butter
1 medium yellow onion, peeled and cut in half
salt, to taste
Combine the tomatoes, their juices, butter and onion halves in a medium saucepan.
Add a pinch or two of salt. Place over medium heat and bring to a simmer. Cook uncovered, stirring occasionally, 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 tomatoes. Mash any large pieces of tomato with the back of a wooden spoon. Taste and salt as needed.
Discard the onion. Serve with spaghetti or your favorite pasta.