Melt the butter in a large sauce pot over medium heat. When butter is melted, quickly stir in the flour to form a smooth roux. Do not brown! Once the roux is smooth and bubbly, slowly pour in the cold milk while stirring briskly to make a smooth sauce. Cook over medium heat, stirring constantly until the sauce thickens. Stir in the lemon juice, Dijon mustard, salt, smoked paprika and white pepper. Bring sauce just back to a low boil and turn off heat. Pour sauce evenly over the casserole, being certain to get some of the sauce around the edges of the pan.