01 - Preheat oven to 375°F. 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 just al dente, about 1-2 minutes less than package directions. Drain thoroughly and set aside.
03 - Steam or blanch broccoli florets for 2-3 minutes until bright green and slightly tender. Drain and set aside.
04 - Melt butter in a large saucepan over medium heat. 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 smooth and thickened. Season with salt, black pepper, and optional pinch of nutmeg.
05 - In a large mixing bowl, combine cooked pasta, diced chicken, broccoli, and prepared Alfredo sauce. Toss to coat evenly.
06 - Transfer the mixture to the prepared baking dish. Sprinkle evenly with shredded mozzarella cheese and 1/4 cup grated Parmesan cheese.
07 - Bake uncovered for 20-25 minutes until the surface is bubbly and golden brown.
08 - Allow casserole to stand for 5 minutes before serving. Garnish with chopped parsley if desired.