01 - Preheat oven to 375°F. Lightly grease a 9x13-inch baking dish.
02 - Bring a large pot of salted water to boil. Add pasta and cook until just al dente, 1-2 minutes less than package directions. Drain well 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 garlic and sauté 1 minute until fragrant. Pour in heavy cream and bring to a gentle simmer. Whisk in 1 cup Parmesan until the sauce is smooth. Season with salt, pepper, and nutmeg if desired.
05 - In a large bowl, mix together cooked pasta, chicken, broccoli, and Alfredo sauce until evenly coated. Spread mixture in prepared baking dish.
06 - Sprinkle shredded mozzarella and an additional 1/4 cup Parmesan evenly over the surface.
07 - Bake for 20-25 minutes, or until casserole is bubbly and golden on top.
08 - Let stand 5 minutes before serving. Garnish with chopped parsley if desired.