01 - Preheat the 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 the pasta and cook until just al dente, about 1–2 minutes less than package instructions. Drain well.
03 - Steam or blanch the chopped broccoli florets for 2–3 minutes until bright green and slightly tender. Drain and set aside.
04 - In a large saucepan, melt the butter over medium heat. Add the minced garlic and sauté for 1 minute until fragrant. Pour in the heavy cream and bring to a gentle simmer. Whisk in 1 cup of grated Parmesan cheese, stirring until the sauce is thick and smooth. Season with salt, black pepper, and a pinch of nutmeg if desired.
05 - In a large mixing bowl, combine the drained pasta, cooked chicken, broccoli, and Alfredo sauce. Stir until all ingredients are evenly coated.
06 - Transfer the mixture to the prepared baking dish. Sprinkle shredded mozzarella and the remaining 1/4 cup Parmesan cheese evenly over the top.
07 - Bake for 20–25 minutes or until the casserole is bubbly and the cheese is golden brown on top.
08 - Remove from the oven and let stand for 5 minutes before serving. Garnish with chopped fresh parsley if desired.