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 well 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.
04 - Pour the heavy cream and whole milk into the skillet, bringing to a gentle simmer. Stir in 1 1/4 cups 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 to a velvety consistency.
05 - Add the shredded chicken and drained pasta to the skillet, tossing thoroughly to coat every piece evenly in the Alfredo sauce.
06 - Transfer the entire mixture into the prepared baking dish, spreading it out into an even layer.
07 - Sprinkle the shredded mozzarella and remaining 1/4 cup Parmesan cheese evenly across the surface to form a uniform crust.
08 - Bake on the center rack for 20 to 25 minutes until the cheese is completely melted, golden brown, and bubbling at the edges.
09 - For an extra-crispy, deeply golden top, switch the oven to broil for 2 to 3 minutes, watching carefully to prevent burning.
10 - Remove from the oven and let rest for 5 minutes to allow the sauce to set. Garnish with chopped fresh parsley and serve warm.