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 chopped onion and minced garlic, sautéing for 2 to 3 minutes until softened and fragrant.
03 - Pour in the heavy cream and milk, stirring frequently. Simmer gently for about 3 minutes without bringing to a full boil.
04 - Stir in the Parmesan cheese, salt, black pepper, and ground nutmeg. Continue cooking for 3 to 4 minutes until the cheese melts and the sauce thickens to a smooth consistency.
05 - In a large mixing bowl, combine the cooked pasta, diced chicken, and Alfredo sauce. Add the dried Italian herbs if using, and toss everything together until evenly coated.
06 - Transfer the mixture into the prepared baking dish, spreading it out evenly and smoothing the top with a spatula.
07 - Sprinkle the shredded mozzarella cheese evenly across the entire surface of the casserole.
08 - Bake uncovered for 25 to 30 minutes, until the mozzarella crust is golden brown and the edges are bubbling.
09 - Remove from the oven and let rest for 5 minutes. Garnish with chopped fresh parsley and serve hot.