Preheat the oven to 375 degrees F (190 degrees C). Grease a 2-quart casserole dish.

Whisk condensed soup, milk, and pepper together in a bowl.

Layer potatoes and onion in the prepared casserole dish. Pour soup mixture over top; it should almost cover potatoes and onion. If it doesn’t, add more milk.

Cover and bake in the preheated 375 degrees F (190 degrees C) oven until potatoes are tender, about 1 hour, stirring halfway through.