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 in the heavy cream and milk, stirring frequently. Simmer gently for about 3 minutes, being careful not to bring it 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 has fully melted and the sauce thickens 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, and toss until everything is evenly coated.
06 - Transfer the mixture to the prepared baking dish, spreading it into an even layer and smoothing the top.
07 - Sprinkle the shredded mozzarella cheese evenly across the entire surface of the casserole.
08 - Bake uncovered for 25–30 minutes, until the mozzarella crust is golden brown and the edges are bubbling.
09 - Allow the casserole to rest for 5 minutes. Garnish with freshly chopped parsley and serve hot.