Preheat the oven to 375°F (190°C) and grease a 9x13-inch baking dish. Set it aside. Cook the penne pasta to al dente following package instructions, then drain and set aside.

Melt the butter in a large saucepan over medium heat. Add the minced garlic and sauté for one minute until fragrant. Stir in the flour and whisk constantly for about two minutes to make a smooth paste.

Gradually pour in the half & half while whisking to combine with the roux. Continue to stir and cook the mixture for 3-5 minutes until it starts to thicken slightly.

Lower the heat and add the grated Parmesan and ½ cup of shredded mozzarella. Stir until the cheeses melt completely and create a smooth, creamy sauce. Mix in the salt and black pepper, then remove from heat.

Combine the cooked pasta and diced chicken with the alfredo sauce in the saucepan or a large bowl. Gently stir until everything is evenly coated.

Transfer the pasta mixture to the prepared baking dish and spread it out into an even layer. Sprinkle the remaining 1 ½ cups of mozzarella evenly over the top.

Cover the dish with aluminum foil and bake for 20 minutes. Then, remove the foil and bake uncovered for an additional 10-15 minutes, or until the cheese on top is melted, bubbly, and slightly golden.

Remove from the oven and garnish with freshly chopped Italian parsley. Serve the casserole hot and enjoy!