Preheat oven to 375 degrees F (190 degrees C). Grease a 2 quart casserole dish.
Layer potatoes and onions into the casserole dish. Combine soup, milk and pepper in a bowl, then pour soup mixture over the potatoes and onions. The soup mixture should almost cover the potatoes and onion, if it does not add extra milk.
Cover dish and bake in preheated 375 degrees F (190 degrees C) oven for 60 minutes or until the potatoes are cooked through. At 30 minutes, remove the casserole from the oven and stir once before returning the dish to the oven. Remove from oven and serve.