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, about 1 to 2 minutes less than package instructions. Drain and set aside.
03 - While pasta cooks, 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. Stir in heavy cream, bring to a gentle simmer, then whisk in 1 cup 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, chicken, broccoli, and Alfredo sauce. Gently mix until ingredients are evenly coated.
06 - Transfer the mixture to the prepared baking dish. Evenly sprinkle mozzarella and 1/4 cup Parmesan cheese over the top.
07 - Bake for 20 to 25 minutes or until casserole is bubbling and cheese is golden brown.
08 - Allow casserole to rest for 5 minutes before serving. Garnish with chopped fresh parsley if desired.