Place potatoes in a large saucepan and cover with water. Bring to a boil. Reduce heat; simmer, uncovered, until tender, 8-10 minutes. Meanwhile, crumble sausage into a large skillet; add onion. Cook over medium heat until meat is no longer pink; drain if necessary., Drain potatoes; arrange in an ungreased 13x9-in. baking dish. Drizzle with butter. Add sausage mixture and stir gently. Sprinkle with cheese., Bake, uncovered, at 350degrees until cheese is melted, 5-7 minutes.