1. Cook fettuccine according to package directions.
2. Meanwhile, in a large skillet, saute chicken in oil until no longer pink. Add mushrooms and onion; saute 2-3 minutes longer or until onion is tender.
3. Sprinkle with flour; cook and stir for 1 minute. Stir in tomatoes and chilies. Bring to a boil. Reduce heat; cover and simmer for 5 - 10 minutes or until thickened.
4. Drain fettuccine; toss with butter. Serve with chicken mixture; sprinkle with cheese if desired.