Put a third of the potato slices into a greased 2 quart casserole. Sprinkle with about a third of onion and a third of blended flour, salt and pepper. Dot with 1 tablespoon of butter. Repeat layering twice, ending with butter. Pour the hot milk over potatoes. Cover casserole. Bake at 350 degrees for 30 minutes. Remove cover; bake 60 to 70 minutes or until potatoes are tender. Remove from oven. Let stand about 5 minutes before serving. Makes about 6 servings.