Combine the butter, flour, milk, salt, and pepper in a saucepan. Bring to a boil, then simmmer. Slice potatoes. Place a layer of potatoes, layer of onions, and cover with sauce until pan is full (or run out of ingrediants). Bake at 350 F covered for a 1/2 hour and uncovered for 1 to 1 1/2 hours. Put something under the pan to prevent sauce getting on the bottom of the oven.