Preheat oven to 120 degrees C. Arrange bread cubes in a single layer on a baking tray. Toast until dry. Crush, and reserve. Increase oven temperature to 180 degrees C.
In a medium bowl, mix together the soup, milk, onion, corned beef and mustard. Set aside.
Spread sauerkraut evenly in the bottom of a lightly greased 20x30cm baking dish. Spread uncooked noodles over sauerkraut. Spoon soup mixture over noodles and sprinkle with cheese. In a small bowl, mix melted butter with rye bread crumbs, and sprinkle mixture over cheese.
Cover and bake in preheated oven for 50 minutes. Remove cover, and bake an additional 10 minutes.