01 - Preheat oven to 375°F. Grease a 9x13-inch baking dish with butter or nonstick cooking 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 unsalted butter. Add minced garlic and sauté for approximately 1 minute until fragrant and lightly golden, stirring constantly to prevent burning.
04 - Pour heavy cream and whole milk into the skillet, bringing the mixture to a gentle simmer. Stir in 1 ¼ cups Parmesan cheese, salt, black pepper, and ground nutmeg. Continue cooking and stirring for 4–5 minutes until the cheese is fully melted and the sauce has thickened enough to coat the back of a spoon.
05 - Add the shredded or diced cooked chicken and drained pasta to the Alfredo sauce. Toss everything together until all components are evenly coated.
06 - Transfer the chicken and pasta mixture to the prepared 9x13-inch baking dish, spreading it into an even layer across the entire surface.
07 - Sprinkle shredded mozzarella cheese and ¼ cup grated Parmesan evenly over the entire surface to form a uniform cheese crust.
08 - Bake on the center rack for 20–25 minutes until the mozzarella crust is fully melted, golden brown, and bubbling at the edges.
09 - For an extra-crispy cheese crust, switch the oven to broil for 2–3 minutes, watching closely to prevent burning.
10 - Remove from oven and let rest for 5 minutes to allow the sauce to set. Garnish with chopped fresh parsley if desired. Serve warm.