01 - Preheat oven to 375°F. Lightly grease a 9x13-inch baking dish and set aside.
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 the package directions. Drain well.
03 - Steam or blanch broccoli florets for 2 to 3 minutes until vivid green and just tender. Drain and reserve.
04 - In a large saucepan over medium heat, melt unsalted butter. Add minced garlic and sauté for 1 minute until aromatic. Stir in heavy cream, bring to a gentle simmer, then whisk in 1 cup Parmesan cheese until smooth. Season with salt, black pepper, and optional nutmeg.
05 - In a large mixing bowl, combine cooked pasta, chicken, broccoli, and Alfredo sauce. Toss thoroughly until all ingredients are coated.
06 - Transfer the mixture to the prepared baking dish. Evenly sprinkle mozzarella and 1/4 cup Parmesan cheese over the top.
07 - Bake in the preheated oven for 20 to 25 minutes until the top is bubbling and golden brown.
08 - Allow casserole to rest for 5 minutes before serving. Garnish with chopped fresh parsley if desired.