Cook fettuccine according to the package directions.
Meanwhile, in a large skillet, saute the chicken, onion, garlic, salt and cayenne in butter until chicken is no longer pink.
Stir in flour until blended.
Gradually add the half-and-half, peas and cheese.
Bring to a boil; cook and stir for 1-2 minutes or until thickened.
Drain fettuccine; toss with chicken mixture.