Preheat oven to 375°F (190°C). Grease a 9x13-inch baking dish.
Cook the pasta: Boil the ziti until just shy of al dente (it’ll finish in the oven). Drain and set aside.
Make the Alfredo sauce: In a large skillet or saucepan, melt butter over medium heat. Add minced garlic and sauté until fragrant (about 30 seconds). Whisk in flour and cook 1–2 minutes to make a roux. Slowly add milk and broth, whisking constantly until smooth. Simmer 3–5 minutes, then stir in Parmesan cheese until melted. Season with salt, pepper, and nutmeg if using. Let thicken slightly.
Assemble: In a large bowl, combine pasta, chicken, half the mozzarella, additional parmesan, and Alfredo sauce. Mix well. Pour into the baking dish, spread evenly. Top with remaining mozzarella.
Bake uncovered for 20–25 minutes, or until bubbly and golden on top. Broil 1–2 minutes at the end if you want a crisp cheesy top.
Garnish with fresh parsley or basil and serve warm!