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 al dente, approximately 1-2 minutes less than package instructions. Drain thoroughly and set aside.
03 - Steam or blanch the broccoli florets for 2-3 minutes until bright green and slightly tender. Immediately drain and set aside to prevent overcooking.
04 - In a large saucepan over medium heat, melt butter. Add minced garlic and sauté for 1 minute until fragrant. Pour in the heavy cream, bring to a gentle simmer, then whisk in 1 cup Parmesan cheese. Stir continuously until the sauce is thickened and smooth. Season with salt, black pepper, and nutmeg if using.
05 - In a large mixing bowl, combine cooked pasta, diced chicken, blanched broccoli, and Alfredo sauce. Toss gently until all pieces are evenly coated.
06 - Transfer the mixture to the prepared baking dish. Evenly sprinkle shredded mozzarella and 1/4 cup grated Parmesan cheese over the surface.
07 - Bake in the preheated oven for 20-25 minutes until bubbly and the top is golden brown.
08 - Allow the bake to stand for 5 minutes before slicing. Garnish with chopped fresh parsley if desired and serve warm.