Place potatoes in a large saucepan and cover with water. Bring to a boil. Reduce the 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 the sausage mixture and stir gently. Sprinkle with cheese. , Bake, uncovered, at 350° until cheese is melted, 5-7 minutes.