Step 1: Cook the Pasta and Season the Chicken
Preheat your oven to 375°F (190°C). Bring a large pot of generously salted water to a rolling boil. Add the pasta and cook according to package directions until it is just al dente (firm to the bite). It will continue to cook in the oven, so you must avoid overcooking it now. Drain and set aside. While the pasta cooks, pat the chicken cubes dry and toss them with olive oil, Italian seasoning, smoked paprika, salt, and pepper.

Step 2: Cook the Chicken
In a large oven-safe skillet or Dutch oven, heat a drizzle of olive oil over medium-high heat. Add the seasoned chicken in a single layer and cook for 5-7 minutes, until cooked through and nicely browned. Remove the chicken from the skillet and set aside.

Step 3: Create the Garlic-Infused Roux
In the same skillet, melt the stick of butter over medium heat. Once melted and foaming, add the minced garlic. Sauté for about 1 minute until it becomes incredibly fragrant but does not brown (browned garlic turns bitter). Sprinkle the flour over the garlic butter and whisk constantly to form a paste. Cook this roux for 2 full minutes to eliminate the raw flour taste. This step is crucial for a properly thickened sauce.

Step 4: Gradually Build the Cream Sauce
Slowly, while whisking constantly, pour in the warm milk and heavy cream. It is vital to add the liquid gradually and whisk vigorously to prevent any lumps from forming. Once smooth, whisk in the onion powder, white pepper, and nutmeg. Bring the sauce to a gentle simmer, stirring frequently, and allow it to thicken for 3-5 minutes until it coats the back of a spoon.

Step 5: Incorporate the Cheeses
Reduce the heat to low. Gradually add 1 ½ cups of the grated Parmesan cheese and the 1 cup of shredded mozzarella cheese to the sauce, a handful at a time, whisking until each addition is completely melted and smooth before adding the next. This patience is the key to a silky, non-grainy sauce. Season the sauce with salt to taste (remember, the Parmesan is salty).

Step 6: Combine All Components
To the luxurious garlic Parmesan sauce, add the cooked al dente pasta and the sautéed chicken cubes. Gently fold everything together until every piece is evenly coated in the creamy sauce.

Step 7: Assemble and Top the Casserole
Transfer the entire mixture to a greased 9×13 inch baking dish. Spread it out into an even layer. In a small bowl, combine the remaining ½ cup of Parmesan cheese, the 1 cup of Italian cheese blend, and the Panko breadcrumbs (if using). Sprinkle this mixture evenly over the top of the casserole.

Step 8: Bake to Golden-Brown Perfection
Place the casserole in the preheated oven and bake, uncovered, for 25-30 minutes, or until the top is a deep golden brown, the sauce is bubbly around the edges, and the topping is crisp.

Step 9: Rest and Serve
Remove the casserole from the oven and let it rest for 10-15 minutes before serving. This resting period is essential for the sauce to thicken up slightly, making serving much cleaner. Garnish with fresh parsley and basil. Serve immediately, reveling in the stretch of the cheese and the burst of garlic aroma.