01 - Preheat oven to 375°F. Grease a 9x13-inch baking dish with butter or nonstick 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. Simmer gently, 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 until the cheese is fully melted and the sauce has thickened to a silky consistency.
05 - In a large mixing bowl, combine the cooked penne, diced chicken, and the prepared Alfredo sauce. Add the dried Italian herbs if using. Toss everything together until evenly coated.
06 - Transfer the mixture to the prepared baking dish, spreading it into an even layer. Sprinkle the shredded mozzarella cheese uniformly across the top.
07 - Bake uncovered for 25–30 minutes, until the mozzarella crust is golden brown and the edges are bubbling.
08 - Remove from the oven and let rest for 5 minutes. Garnish with freshly chopped parsley and serve hot.