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 al dente, 1 to 2 minutes less than package instructions. Drain and set aside.
03 - Steam or blanch broccoli florets for 2 to 3 minutes until bright green and slightly tender. Drain and set aside.
04 - In a large saucepan over medium heat, melt butter. Add minced garlic and sauté for 1 minute until fragrant. Pour in heavy cream and bring to a gentle simmer. Whisk in 1 cup grated Parmesan cheese until the sauce is thick and smooth. Season with salt, black pepper, and optional nutmeg.
05 - In a large mixing bowl, combine cooked pasta, diced chicken, steamed broccoli, and Alfredo sauce. Toss gently to evenly coat all components.
06 - Transfer mixture into the prepared baking dish. Sprinkle mozzarella and 1/4 cup Parmesan cheese evenly over the top.
07 - Bake uncovered for 20 to 25 minutes until casserole is bubbling and top is golden.
08 - Remove from oven and allow to stand for 5 minutes before serving. Garnish with chopped fresh parsley if desired.