01 - Preheat 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 penne or rotini pasta and cook until just al dente, approximately 1-2 minutes less than package instructions. Drain thoroughly.
03 - While pasta cooks, steam or blanch broccoli florets for 2-3 minutes until vibrant green and slightly tender. Drain and set aside.
04 - Melt butter in a large saucepan over medium heat. Add minced garlic and sauté for 1 minute until aromatic. Pour in heavy cream, bring to a gentle simmer, then whisk in Parmesan cheese until sauce is thickened and smooth. Season with salt, black pepper, and nutmeg if desired.
05 - In a large mixing bowl, combine cooked pasta, diced or shredded chicken breast, broccoli florets, and Alfredo sauce. Toss until evenly coated.
06 - Transfer mixture to the prepared baking dish. Evenly sprinkle shredded mozzarella and remaining Parmesan cheese over the top.
07 - Place dish in the oven and bake for 20-25 minutes until bubbly and golden on top.
08 - Allow casserole to rest for 5 minutes before portioning. Garnish with chopped fresh parsley if desired and serve warm.