Sprinkle a few onions on the bottom of pan...adding a layer of potatoes, sprinkling with salt & pepper. After layering half the potatoes. Sprinkle with half the onions. Lay cheese slices over top. Layer the second layer of potatoes (sprinkling salt & pepper to season) Sprinkle remaining onions over the top.
Heat milk to boiling (I use the microwave). Add the cream of mushroom soup...blending together well. Pour over potatoes.
Cover tightly with foil. Bake in 350' oven for 1 hour. Uncover foil, and continue to bake another 30 minutes.