01 - Preheat oven to 375°F. Grease a 9x13-inch baking dish with butter or non-stick spray.
02 - In a large skillet over medium heat, melt the butter. Add the finely chopped onion and minced garlic, sautéing for 2–3 minutes until softened and fragrant.
03 - Pour the heavy cream and milk into the skillet with the aromatics. Bring to a gentle simmer, stirring frequently, for about 3 minutes. Do not allow the mixture to come to a full boil.
04 - Stir in the grated Parmesan cheese, salt, black pepper, and ground nutmeg. Continue cooking for 3–4 minutes, stirring constantly, until the cheese is fully melted and the sauce has thickened to a creamy consistency.
05 - In a large mixing bowl, combine the cooked penne or rigatoni, diced or shredded chicken, and the prepared Alfredo sauce. Add the dried Italian herbs if using. Toss everything together until evenly coated.
06 - Transfer the chicken and pasta mixture to the prepared 9x13-inch baking dish, spreading it out into an even layer and smoothing the top.
07 - Sprinkle the shredded mozzarella cheese evenly across the entire surface of the casserole, ensuring full coverage for a uniform golden crust.
08 - Bake uncovered on the center rack for 25–30 minutes, until the mozzarella topping is melted, golden brown, and bubbling at the edges.
09 - Remove from the oven and allow the casserole to rest for 5 minutes. Garnish with freshly chopped parsley, then serve hot directly from the baking dish.