Move the oven rack to the middle position and preheat the oven to 400 degrees F.
Wash the potatoes under cold water, and peel and cut them into 1/8’ slices. Lay half of the potatoes flat in a buttered 2-quart baking dish. Set the remaining potatoes aside.
In a large measuring cup add the milk, mustard powder, salt, and pepper and set aside.
In a medium saucepan over medium heat melt the butter. Add the flour a cook, stirring constantly, for 3 minutes. Slowly whisk in the milk mixture and continue whisking until the mixture is thickened and bubbly about 3-4 minutes. Remove the pan from the heat and whisk in the cheese until melted and velvety. Season to taste with salt and pepper.
Pour half of the sauce over the potatoes, layer the rest of the potatoes on top of the sauce, and cover the potatoes with the remaining sauce. Sprinkle the reserved cheddar cheese over the sauce.
Spray a piece of foil with nonstick cooking spray and cover the dish with foil. Bake for 60 minutes, and then remove the foil and bake an additional 20-30 minutes or light golden brown in color and the potatoes are cooked through.
Cool on a wire rack for 5 minutes, and then garnish with freshly chopped parsley or chives (optional), and serve.