Place potatoes in a saucepan and cover with water. Bring to a boil. Reduce heat; cover and cook for 12 minutes or until potatoes are almost tender. Drain. In a bowl, combine soup and milk. Stir in cheese and pepper.
In a greased shallow 1-qt. baking dish, layer half of potatoes and half of soup mixture. Repeat layer. Bake, uncovered, at 350° for 20 minutes. Sprinkle with onions. Bake 5-10 minutes longer or until bubbly and onions are golden brown.