01 - Preheat oven to 375°F. Grease a 9x13-inch baking dish with nonstick spray or butter.
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 the package instructions. Drain and set aside.
03 - Steam or blanch broccoli florets for 2-3 minutes until bright green and just tender. Drain thoroughly and set aside.
04 - In a large saucepan over medium heat, melt butter. Add minced garlic and sauté for 1 minute until aromatic.
05 - Pour in the heavy cream and bring to a gentle simmer. Whisk in 1 cup grated Parmesan cheese until sauce is velvety and thick. Season with salt, black pepper, and nutmeg if desired.
06 - In a large mixing bowl, combine cooked pasta, diced chicken, broccoli, and Alfredo sauce. Mix thoroughly until all ingredients are coated.
07 - Transfer pasta mixture to the prepared baking dish. Evenly sprinkle shredded mozzarella and 1/4 cup Parmesan cheese over the top.
08 - Bake for 20-25 minutes or until the top is golden and the casserole is bubbling.
09 - Let stand for 5 minutes before serving. Garnish with chopped fresh parsley if desired.