First of all, you have to take a large skillet and place it over medium heat. Now put chicken in the skillet before adding in 2 tablespoons butter, 2 cloves garlic and Italian seasoning. Cook for few minutes, until you find that the pinkness of the chicken has completely disappeared. Remove chicken from the skillet and set aside.
Now take a large pot and fill it with water. Add in some salt and place the pot over high heat. Bring the water to boil before adding in pasta. Cook for around 10 minutes. Drain the water and set pasta aside.
Take another skillet and melt 4 tablespoons butter in it. Sauté onion, 2 cloves garlic, and mushrooms for few minutes, until the onions turn transparent. Now you can stir in flour, salt and pepper and cook the mixture for not more than 2 minutes. Gradually add in milk and Half-and-Half and wait until the mixture becomes smooth and creamy. The consistency of this mixture is very important for the overall texture of this dish.
Stir in Parmesan and Colby-Monterey Jack cheeses and wait for the cheese to completely melt. You can now add in the chicken mixture that you had created in the first step, along with tomatoes and sour cream. Take it out in a serving bowl, pouring over cooked fettuccini.