Heat a heavy, medium saucepan over medium heat. Add all of the ingredients and bring to a simmer. Turn the heat to low to keep a steady simmer. Cook for 45 minutes, or until droplets of fat float free of the tomatoes. Stir occasionally. Discard the onion. Taste and adjust seasoning. Serve over cooked pasta. recipe adapted from Essentials of Classic Italian Cooking by Marcella Hazan http://steamykitchen.com/8375-marcella-hazan-tomato-sauce-with-onion-butter.html