In a 9x13 pan, layer half the potatoes and half the onion. Sprinkle half the flour and half the salt. Grind some pepper over it. Cover with half the milk. Repeat all layers. Milk should come right up to the top of the potatoes. Bake covered at 350º for 35 minutes. Uncover and spoon the sauce over the top of the potatoes and then bake for 10 more minutes, uncovered.
Cut into 12 equal parts.