Heat oven to 350 F. Wash potatoes; pare thinly and remove eyes. Cut potatoes into thin slices to measure about 4 cups. In greased 2-quart casserole, arrange potatoes in 4 layers, sprinkling each of the first 3 layers with 1 tablespoon onion, 1 tablespoon flour, 1/4 teaspoon salt, dash pepper and dotting each with 1 tablespoon butter. Sprinkle top with remaining onion, salt and pepper and dot with remaining butter. Heat milk just to scalding; pour over potatoes. Cover; bake 30 minutes. Uncover; bake 80 to 70 minutes longer or until potatoes are tender. Let stand 5 to 10 minutes before serving. 4 TO 6 SERVINGS.