Preheat oven to 425F (220C). Spray your largest baking dish (I used a deep 9 x 13-inch dish) with cooking spray.
In the prepared dish (or in a separate bowl), whisk together the Alfredo sauce, chicken broth, and sour cream until smooth and completely combined. Stir in the uncooked noodles, onion, turkey, mushrooms, and garlic. Cover tightly with aluminum foil and bake for 30 minutes.
Uncover; stir. At this point you should check the pasta to make sure that it is al dente (firm but just about finished cooking). If it’s still too hard, cover the dish and return to the oven until pasta is al dente. Then move on to the next step.
Sprinkle cheese over top of the casserole and bake, uncovered, for 10 more minutes (or until cheese is melted and pasta is tender). Garnish with parsley, if desired.
Notes
Cooking Just for Two? You can prepare a smaller casserole by cutting all of the ingredients in half and baking the tetrazzini in an 8-inch square dish.