01 - Preheat oven to 200°C (400°F). Lightly grease a 23 x 33 cm (9 x 13-inch) baking dish.
02 - Bring a large pot of salted water to a boil. Cook pasta according to package instructions until al dente. Drain and set aside.
03 - While pasta cooks, heat olive oil in a large skillet over medium-high heat. Add diced chicken, season with salt and pepper. Sauté until cooked through and lightly golden, approximately 5–7 minutes. Remove chicken from skillet and set aside.
04 - In the same skillet, melt butter over medium heat. Add minced garlic and sauté for 1 minute until fragrant.
05 - Sprinkle flour over the sautéed garlic and whisk continuously for 1 minute to form a roux.
06 - Gradually incorporate milk and heavy cream into the skillet, whisking constantly to achieve a smooth consistency. Simmer for 3–4 minutes until the sauce begins to thicken.
07 - Stir in Parmesan cheese, salt, nutmeg (if using), and black pepper. Continue to cook for another 2 minutes until the cheese is melted and the sauce is creamy.
08 - Add the cooked chicken and drained pasta to the Alfredo sauce. Toss gently until all components are evenly coated.
09 - Transfer the combined mixture into the prepared baking dish. Distribute shredded mozzarella and additional grated Parmesan cheese evenly over the top.
10 - Place the baking dish in the preheated oven and bake for 20 minutes, or until the cheese is melted and the top is lightly golden.
11 - Allow the bake to rest for 5 minutes before serving. Garnish with chopped fresh parsley, if desired.