Preheat the oven to 350° F/180° C. Butter or oil a small baking dish (I used a 4 1⁄2 x 7-inch/close to 20 x 10 cm casserole).
Put a layer of the raw potatoes in the baking dish. Sprinkle salt and pepper on top, dot the butter, and sprinkle or sift about a 1⁄2–1 tablespoon of flour on top. (If desired, add the shredded cheese mixed with dry mustard.) Repeat the layers until all the potatoes are used up. End with butter.
Pour milk into the dish until it just about covers the top potatoes. Bake for about 45 minutes covered, then uncover and bake for an additional 15 minutes. If desired, broil for 1–2 minutes at the end to crisp up the top layer of potatoes.