Step 1: Cook the pasta according to package directions until al dente. Drain and set aside.
Step 2: While pasta cooks, heat olive oil in a large skillet or Dutch oven over medium heat. Add chopped onion and sauté until softened, about 5 minutes. Add minced garlic and cook for another minute until fragrant.
Step 3: Add butter to the skillet and let it melt. Sprinkle in the flour and whisk constantly for 1-2 minutes to create a roux.
Step 4: Gradually whisk in the chicken broth, then the milk, ensuring no lumps. Bring the sauce to a simmer, stirring continuously, until it thickens.
Step 5: Reduce heat to low, then stir in the softened cream cheese until fully melted and smooth. Add Parmesan cheese, Italian seasoning, salt, and pepper. Stir until well combined.
Step 6: Remove the skillet from heat. Stir in the cooked chicken and the drained pasta. Add 1.5 cups of the shredded mozzarella cheese and mix until everything is evenly coated.
Step 7: Transfer the pasta mixture to a 9x13 inch baking dish. Sprinkle the remaining 1/2 cup of mozzarella cheese over the top.
Step 8: Bake in a preheated oven at 375°F (190°C) for 20-25 minutes, or until the cheese is melted and bubbly and the sauce is heated through. For a golden-brown crust, you can broil for the last 2-3 minutes, watching carefully.
Step 9: Remove from oven and let stand for 5-10 minutes before serving. Garnish with fresh parsley, if desired.