Tomato Sauce with Butter and Onion

Add the halved onion, butter, tomatoes and their juices in a saucepan (I found that everything fit nicely in a 2-quart saucepan). Uncovered, over medium heat bring the sauce to a simmer then adjust the heat to keep the sauce at a slow, steady simmer for about 45 minutes, or until the sauce thickens and droplets of fat float free of the tomatoes. Stir occasionally, breaking up the tomatoes with a spoon against the side of the pot. Discard onion, and season with salt if needed. Toss with prepared pasta and serve.