Preheat the oven to 375 degrees F. Lightly butter a 9" x 13" baking dish. Place the fish in a single layer in the dish. Pour the milk into a small pot and whisk in the flour until smooth. Set over medium heat and bring to a simmer. Whisk in the cheese, paprika, cayenne, salt and white pepper. Pour the cheese sauce over the fish. Sprinkle the fish with Parmesan cheese. Bake, uncovered, for 15 to 18 minutes, or until the fish is golden on top and cooked through. Sprinkle with parsley and serve.