Tomato Sauce with Onion and Butter

28 ounces whole peeled tomatoes from a can (San Marzano, if you can find them)
5 tablespoons unsalted butter
1 medium-sized yellow onion, peeled and halved
Salt to taste

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) and keep warm while you prepare your pasta.

Serve with spaghetti, with or without grated parmesan cheese to pass.