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 to 3 minutes until softened and fragrant.
03 - Pour in the heavy cream and milk, stirring frequently. Bring to a gentle simmer and cook for about 3 minutes, being careful not to let it reach a full boil.
04 - Stir in the grated Parmesan cheese, salt, black pepper, and ground nutmeg. Continue cooking for 3 to 4 minutes until the cheese has 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. If using dried Italian herbs, add them now. Toss everything together until evenly coated.
06 - Transfer the entire mixture into the prepared 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.
08 - Bake uncovered for 25 to 30 minutes, until the mozzarella crust is golden brown and the sauce is bubbling around the edges.
09 - Remove from the oven and let rest briefly for 5 minutes. Garnish with freshly chopped parsley and serve hot.