Preheat the oven to 375°F. Grease an 9-by-12-inch baking dish. Peel the potatoes and slice them ¼-inch thick. Place half the potatoes in the baking dish. Add half the cheese if you are using it. Sprinkle with 1 tablespoon flour, 2 tablespoons butter, and ½ teaspoon salt. Add the remaining potatoes and sprinkle them with the remaining flour, butter, salt, pepper, and cheese if you are using it. Pour the milk over everything.

Cover with foil and bake for 45 minutes. Remove the foil and bake for 40 minutes longer. Serve hot.