In a large skillet over medium heat combine chicken, garlic, 2 tablespoons butter, and Italian seasoning. Cook until chicken is no longer pink inside. Remove from skillet and set aside. Cook pasta according to package directions, or in boiling water 8-10 minutes or until al dente
. Meanwhile, melt 4 tablespoons butter in the skillet. Saute onion, 2 tablespoons garlic 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 cheeses; stir until cheese is melted. Stir in chicken mixture, tomatoes and sour cream. Serve over cooked fettuccini. Serves 8.