01 - Preheat oven to 375°F. 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 minced garlic and sauté for approximately 1 minute until fragrant, being careful not to brown.
04 - Pour the heavy cream and whole milk into the skillet, bringing to a gentle simmer. Stir in 1 ¼ cups Parmesan cheese, salt, black pepper, and ground nutmeg. Continue stirring for 4–5 minutes until the cheese is fully melted and the sauce has thickened to a silky consistency.
05 - Add the shredded or diced cooked chicken and drained pasta to the Alfredo sauce. Toss thoroughly until all ingredients are evenly coated.
06 - Transfer the entire mixture into the prepared 9x13-inch baking dish, spreading it out into an even layer.
07 - Sprinkle the shredded mozzarella cheese and ¼ cup grated Parmesan evenly across the entire surface to form the cheese crust.
08 - Bake in the preheated oven for 20–25 minutes until the cheese is melted, golden, and bubbling. For a crispier, more caramelized crust, broil for an additional 2–3 minutes, watching closely to prevent burning.
09 - Remove from the oven and let rest for 5 minutes to allow the sauce to set. Garnish with chopped fresh parsley if desired, then serve warm.