Preheat your oven to 375°F (190°C). Grease a 9x13-inch baking dish.
Cook the penne pasta in salted water until just al dente (about 2 minutes less than package directions). Drain and set aside.
In a large saucepan, melt the butter over medium heat. Add minced garlic and cook until fragrant, about 1 minute.
Whisk in the flour and cook for 1-2 minutes to remove the raw flour taste.
Gradually pour in the milk and cream, whisking constantly to prevent lumps.
Simmer until the sauce thickens, about 5 minutes, stirring occasionally.
Remove from heat and stir in Parmesan cheese, ½ cup mozzarella, Italian seasoning, salt, and pepper.
Combine the cooked pasta, diced chicken, and Alfredo sauce in a large bowl.
Transfer to the prepared baking dish and top with remaining mozzarella.
Bake for 20-25 minutes until bubbly and golden on top.
Let rest for 5 minutes before serving. Garnish with fresh parsley.