Preheat oven to 425 degrees F (220 degrees C). Spray a 9 x 13-inch baking dish with cooking spray.
In the prepared baking dish (or in a separate bowl), whisk together Alfredo sauce and chicken broth until completely combined.
Stir in the uncooked pasta, ½ cup of the cheese, chicken, parsley and garlic.
Cover the dish tightly with foil. Bake for 35 minutes.
Uncover; stir. At this point, the pasta should be al dente (firm but just about finished cooking). There will still be plenty of liquid in the dish, but don’t worry – this will thicken as the dish continues to bake and cool.
If the pasta is still too hard after 35 minutes, just cover the dish and return to the oven until pasta is al dente. Then move on to the next step.
Sprinkle remaining ½ cup parmesan over top. Bake, uncovered, for 10 more minutes (or until cheese is melted and pasta is tender). Give it another stir, and allow to rest for about 5-10 minutes before serving (the sauce will continue to thicken as it cools).
Garnish fresh herbs, if desired.