Peel potatoes, rinse and cut into 1/8 inch thick slices (should be about 4 cups.
Sprinkle 1 1/2 tablespoons flour on bottom of greased casserole.
Arrange 1/2 the potatoes and onions evenly in casserole.
Sprinkle 1 1/2 tablespoons flour on potatoes.
Arrange the remaining potatoes and onions evenly on top of the first layer. Add salt and pepper. Cut the butter into pats and place evenly on top of the potatoes. Pour milk over the top.
Cover and cook in 325° F. oven 40 minutes. Uncover and cook until potatoes are tender, 60 to 70 minuters longer.
Let stand 5 to 10 minutes before serving.