Chicken Fettuccine Alfredo

6 skinless, boneless chicken breast halves - cut into cubes
6 tablespoons butter, divided
4 cloves garlic, minced, divided
1 tablespoon Italian seasoning
1 pound fettuccine pasta
1 onion, diced
1 (8 ounce) package sliced mushrooms
1/3 cup all-purpose flour
1 tablespoon salt
3/4 teaspoon ground white pepper
3 cups milk
1 cup half-and-half
3/4 cup grated Parmesan cheese
8 ounces shredded Colby-Monterey Jack cheese
3 (plum) tomatoes, diced
1/2 cup sour cream

In a large skillet over medium heat combine chicken, 2 tablespoons butter, 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. Sauté onion, 2 tablespoons 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 fettuccine.