combine chicken, 2 tablespoons butter, 2 cloves garlic, and Italian seasoning in large skillet over medium heat

when chicken is no longer pink inside remove and set aside

bring large pot of lightly salted water to a boil

add pasta and cook for 8-10 minutes

drain pasta when cooked

meanwhile melt 4 tablespoons butter in the skillet

saute onion, 2 cloves garlic, and mushrooms until onions transparent.

stir in flour, salt, and pepper; cook for 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