01 - Preheat oven to 375°F. Lightly grease a 9x13-inch baking dish.
02 - Bring a large pot of salted water to a boil. Add pasta and cook until just al dente, about 1-2 minutes less than package directions. Drain and set aside.
03 - Steam or blanch broccoli florets for 2-3 minutes until bright green and slightly tender. Drain and set aside.
04 - In a large saucepan, melt butter over medium heat. Add minced garlic and sauté for 1 minute until fragrant. Pour in the heavy cream, bring to a gentle simmer, then whisk in 1 cup Parmesan cheese until sauce is thick and smooth. Season with salt, black pepper, and nutmeg if using.
05 - In a large mixing bowl, combine cooked pasta, chicken, broccoli, and Alfredo sauce. Stir until thoroughly coated.
06 - Transfer the mixture to the prepared baking dish. Sprinkle mozzarella cheese and 1/4 cup Parmesan cheese evenly over the top.
07 - Bake in the oven for 20-25 minutes or until the top is golden and the casserole is bubbling.
08 - Let stand for 5 minutes before serving. Garnish with chopped parsley if desired.