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.