Step 1: Cook the Pasta and Prepare the Chicken
Preheat your oven to 375°F (190°C). Bring a large pot of salted water to a boil and cook the pasta 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 season them with salt, pepper, and smoked paprika. In a large skillet, heat the olive oil over medium-high heat. Add the chicken in a single layer and cook for 5-7 minutes, until golden brown and cooked through. Remove the chicken from the skillet and set aside.

Step 2: Create the Garlic-Infused Roux
In the same skillet or a large Dutch oven, melt the stick of butter over medium heat. Once melted, add the minced garlic and sauté for about 1 minute until it becomes incredibly fragrant. Do not let it brown, as this will make it 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.

Step 3: Gradually Build the Cream Sauce
Slowly, while whisking constantly, pour in the warm milk and chicken broth. It is crucial to add the liquid gradually and whisk vigorously to prevent any lumps from forming. Once smooth, whisk in the heavy cream, Italian seasoning, and onion powder. 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 4: 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 additional salt and pepper to taste.

Step 5: Combine All Components
To the luxurious garlic Parmesan sauce, add the cooked al dente pasta, the sautéed chicken cubes, and your chosen vegetable (the squeezed-dry spinach or blanched broccoli). Gently fold everything together until every component is evenly coated in the creamy sauce.

Step 6: 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 7: Bake to Golden-Brown Perfection
Place the casserole in the preheated oven and bake, uncovered, for 25-30 minutes, or until the top is golden brown and the sauce is bubbly around the edges. If you desire an extra-crispy top, you can broil it for the final 1-2 minutes, watching carefully to prevent burning.

Step 8: 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 chopped parsley and serve immediately