Preheat your oven to 375°F (190°C). Lightly grease a 9x13 inch casserole dish.

If using fresh broccoli, steam or blanch it for 2-3 minutes until bright green and slightly tender-crisp. Drain very well and pat dry to remove excess water. This step is crucial to prevent a watery casserole.

Arrange the cooked turkey and prepared broccoli florets in an even layer in the bottom of the prepared casserole dish.

In a large saucepan over medium heat, melt 4 tablespoons of butter. Whisk in the flour and cook for 1 minute, stirring constantly, to create a smooth roux.

Gradually pour in the warm milk and chicken broth while whisking constantly to prevent lumps. Continue to cook, whisking frequently, for 5-7 minutes until the sauce thickens enough to coat the back of a spoon.

Remove the saucepan from the heat. Stir in 1.5 cups of the shredded cheddar cheese (reserving 1/2 cup for the top), Dijon mustard, garlic powder, onion powder, salt, and pepper. Continue stirring until the cheese is completely melted and the sauce is velvety smooth.

Pour the cheese sauce evenly over the turkey and broccoli in the casserole dish. Gently stir to ensure everything is well-coated. Sprinkle the remaining 1/2 cup of shredded cheddar cheese over the top.

In a small bowl, combine the Panko breadcrumbs, 2 tablespoons of melted butter, grated Parmesan cheese, and paprika. Use a fork to toss until the breadcrumbs are evenly moistened.

Sprinkle the breadcrumb mixture evenly over the casserole.

Bake for 25-30 minutes, uncovered, until the sauce is hot and bubbly and the topping is golden brown and crispy. Let the casserole rest for 5-10 minutes before serving to allow it to set.