Preheat oven to 250 degrees F (120 degrees C). Arrange bread cubes in a single layer on a baking sheet. Toast until dry. Crush, and reserve. Increase oven temperature to 350 degrees F (175 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 9x13 inch 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.