In a large saucepan set over moderate heat, cook the onion in the oil, stirring occasionally, for 5 minutes. Add the sausage and cook it until no longer pink. Drain, then add the tomato sauce, peas and liquid, water, salt, and pepper, and basil. Bring to a boil and simmer for 20 minutes.
Meanwhile, in a large saucepan of boiling salted water, cook the macaroni until barely tender, drain, and add to the sauce. Simmer until heated through. Let stand for 5 minutes before serving. Serve with the Parmesan.
Recipe courtesy of Catherine Scorcese, Italianamerican: The Scorcese Famly Cookbook, 1996