01 -  Preheat oven to 375°F. Grease a 9x13-inch baking dish. 
 02 -  Boil macaroni according to package directions until al dente. Drain and set aside. 
 03 -  Melt 4 tablespoons unsalted butter in a large saucepan over medium heat. Add minced garlic and sauté for 1–2 minutes until aromatic. 
 04 -  Pour in heavy cream and whole milk. Season with salt, black pepper, and nutmeg. Heat gently to a simmer without boiling. 
 05 -  Reduce heat to low and whisk in mozzarella, cheddar, and Parmesan cheese until completely melted and smooth. 
 06 -  Stir in cooked macaroni and chicken breast, ensuring all pieces are evenly coated with the sauce. 
 07 -  Pour the mixture into the prepared baking dish, spreading evenly. 
 08 -  Mix panko breadcrumbs, melted butter, and grated Parmesan cheese together in a bowl. Sprinkle uniformly over the casserole. 
 09 -  Bake uncovered for 20–25 minutes until the topping is golden brown and the sauce is bubbling at the edges. 
 10 -  Remove from the oven and allow to rest for 5 minutes. Garnish with chopped fresh parsley prior to serving as preferred.