01 - Preheat oven to 375°F. Grease a 9x13-inch baking dish with a thin layer of butter or nonstick spray.
02 - Bring a large pot of salted water to a boil. Add pasta and cook until just al dente, approximately 1-2 minutes less than package instructions. Drain well and set aside.
03 - Steam or blanch broccoli florets for 2–3 minutes until they turn bright green and remain slightly firm. Drain promptly to prevent overcooking.
04 - In a large saucepan, melt butter over medium heat. Add garlic and sauté for 1 minute until fragrant. Pour in heavy cream and bring to a gentle simmer. Gradually whisk in 1 cup Parmesan until the sauce is smooth and thickened. Season with salt, black pepper, and nutmeg if using.
05 - In a large mixing bowl, gently fold together the cooked pasta, diced chicken, broccoli, and Alfredo sauce until evenly incorporated.
06 - Transfer mixture to the prepared baking dish and spread evenly. Top with shredded mozzarella and 1/4 cup Parmesan, distributing cheese over the surface.
07 - Place baking dish in the oven and bake for 20–25 minutes until casserole is bubbly and the top is golden brown.
08 - Remove from oven and allow to stand for 5 minutes before serving. Garnish with chopped fresh parsley if desired.