oil in a medium sized, flameproof casserole over medium heat; when the oil is warm, add the onion and saute until the outer layers are translucent, about 5 minutes. Add the tomatoes to the casserole and cook for 25 minutes, seasoning to taste with salt and pepper. Remove and discard the onion, then pass the contents through a food mill, using the disc with the smallest holes, into a second flameproof casserole. Place the second casserole over low heat and reduce sauce for 10 minutes.

As the pasta cooks tear the basil leaves into thirds and add them to the sauce.