In a large skillet over medium heat combine chicken, 2 tablespoons butter, 2 cloves garlic, and Italian seasoning. Cook until chicken is no longer pink inside. Remove from skillet and set aside.
Bring a large pot of lightly salted water to a boil. Add pasta and cook for 8 to 10 minutes or until al dente; drain.
Meanwhile, melt 4 tablespoons butter in the skillet. Saute onion, 2 cloves garlic, and mushrooms until onions are transparent. Stir in flour, salt and pepper; cook 2 minutes. Slowly add milk and half-and-half, stirring until smooth and creamy. Stir in Parmesan and Colby-Monterey Jack cheeses; stir until cheese is melted. Stir in chicken mixture, tomatoes and sour cream. Serve over cooked fettuccini.