01 - Preheat the oven to 375°F. Lightly 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 to 2 minutes shy of package instructions. Drain well.
03 - Steam or blanch broccoli florets for 2 to 3 minutes until bright green and slightly tender. Drain broccoli and set aside.
04 - In a large saucepan, melt butter over medium heat. Add minced garlic and sauté for about 1 minute until fragrant. Pour in heavy cream and bring to a gentle simmer. Whisk in 1 cup Parmesan cheese until the sauce is thickened and smooth. Season with salt, black pepper, and nutmeg if using.
05 - In a large mixing bowl, gently fold together cooked pasta, chicken, broccoli, and Alfredo sauce, ensuring everything is evenly coated.
06 - Transfer the mixture to the prepared baking dish. Evenly sprinkle mozzarella and 1/4 cup Parmesan cheese over the top.
07 - Bake uncovered for 20 to 25 minutes until the dish is bubbly and the cheese is golden brown.
08 - Allow the casserole to stand for 5 minutes before serving. Garnish with fresh parsley if desired.