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 instructions. Drain well and set aside.
03 - Steam or blanch broccoli florets for 2-3 minutes until bright green and slightly tender, then 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 Parmesan cheese until the sauce is smooth and thickened. Season with salt, black pepper, and optional nutmeg.
05 - In a large mixing bowl, toss together cooked pasta, diced chicken breast, broccoli, and prepared Alfredo sauce until all components are thoroughly coated.
06 - Distribute the mixture evenly in the prepared baking dish. Sprinkle shredded mozzarella and 1/4 cup grated Parmesan over the top.
07 - Bake uncovered for 20-25 minutes or until the casserole is bubbling and golden brown on top.
08 - Allow to stand for 5 minutes before serving. Finish with chopped fresh parsley if desired.