Set your oven to 375 degrees and lightly grease a 3-quart baking dish with cooking spray.
In a large skillet, melt the butter over medium heat then stir in the garlic, cooking it for about 30 seconds.
Add the flour and whisk continuously for 1 minute to form a roux.
Slowly pour in the whole milk and heavy cream, stirring constantly and letting the sauce simmer gently until it thickens.
Mix in parmesan cheese and half a cup of shredded mozzarella, then season with salt and pepper to your taste.
Combine the cooked penne pasta and diced chicken in a large bowl.
Pour the creamy sauce over the pasta and chicken mixture and toss everything until well coated.
Transfer this mix into your prepared baking dish and sprinkle the remaining mozzarella evenly on top.
Bake uncovered for around 20 minutes, or until you see the cheese bubbling and turning golden brown.
Finish by sprinkling chopped parsley over the casserole and serve warm.