01 - Preheat oven to 375°F. Lightly grease a 9x13-inch baking dish with butter or nonstick spray.
02 - Bring a large pot of salted water to a rolling boil. Cook penne or rigatoni according to package directions until al dente. Drain thoroughly and set aside.
03 - In a large skillet over medium heat, melt the butter. Add the minced garlic and sauté for approximately 1 minute until fragrant and lightly golden, being careful not to burn.
04 - Pour the heavy cream and whole milk into the skillet, bringing to a gentle simmer. Stir in 1¼ cups of Parmesan cheese, salt, black pepper, and nutmeg. Continue cooking and stirring for 4 to 5 minutes until the cheese is fully melted and the sauce thickens enough to coat the back of a spoon.
05 - Add the shredded chicken and drained pasta to the skillet, tossing everything together until evenly coated with the Alfredo sauce.
06 - Transfer the mixture to the prepared baking dish, spreading it into an even layer. Sprinkle the mozzarella and remaining ¼ cup of Parmesan evenly across the top to form the crust.
07 - Bake on the center rack for 20 to 25 minutes until the cheese is fully melted, golden in spots, and bubbling around the edges. For a crispier, more caramelized crust, broil for an additional 2 to 3 minutes, watching closely to prevent burning.
08 - Remove from the oven and let rest for 5 minutes to allow the sauce to set. Garnish with chopped fresh parsley before serving.