Put the tomatoes, onion and butter in a heavy saucepan (it fit just right in a 3-quart) over medium heat. Bring the sauce to a simmer then lower the heat to keep the sauce at a slow, steady simmer for about 45 minutes, or until droplets of fat float free of the tomatoes. Stir occasionally, crushing the tomatoes against the side of the pot with a wooden spoon. Remove from heat, discard the onion, add salt to taste (you might find, as I did, that your tomatoes came salted and that you didn’t need to add more)