In a large skillet over medium heat combine chicken, half of the minced garlic cloves, 2 tablespoons of butter, and Italian seasoning. Cook chicken until its no longer pink inside. Remove from the pan and set aside.
Cook fettuccini according to package directions.
While fettuccine is cooking melt 4 tablespoons of butter in your skillet. Sauté onion, 2 tablespoons of garlic, and mushrooms until the onions are transparent. Stir in the flour, salt, and pepper, and cook for a couple minutes. Slowly add the milk, half and half, and stir until it’s creamy and smooth. Stir in the Colby-jack and parmesan cheeses, and stir until cheese is melted. Stir in the chicken mixture, diced tomatoes, and sour cream. Serve over fettuccini.