Prep the oven and baking dish. Heat the oven to 350°F and lightly grease a baking dish.

Sauté the shallots. Melt 2 tablespoons of butter in a medium skillet over medium heat. Add the shallots and sauté until softened and translucent.

Make the spinach stuffing. Add the cooked spinach, 1/2 teaspoon of kosher salt, Italian seasoning, garlic powder, and 1/8 teaspoon of pepper to the skillet with the shallots. Taste for seasoning. Combine and set aside.

Stuff the fish. Place 1 1/2 to 2 tablespoons of the spinach mixture on a flounder fillet. Roll it up and place it seam-side down in the baking dish. repeat with the remaining fish and spinach. Add the wine to the baking dish and bake the flounder rolls for 25 minutes.

Make the sauce. While the fish is baking, melt the remaining 3 tablespoons of butter in a saucepan over medium-low heat. When the butter begins to sizzle, add the flour, remaining salt, remaining pepper, and mustard powder. Cook, stirring, for 2 minutes. Gradually add the milk, stirring constantly. Cook for 2 to 3 minutes, or until the sauce has thickened. Then, stir in the Parmesan cheese and continue stirring until it has melted. Taste and adjust the seasonings as needed.

Finish the dish. Pour the sauce over the stuffed flounder rolls and sprinkle with paprika just before serving.