1. Preheat oven to 350 degrees. Toast slices of rye in toaster until dry. Crush, and reserve for later.
2. In a medium bowl, mix together the soup, milk, onion, corned beef, and mustard. Set aside.
3. 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 that over the cheese.
4. Cover and bake for 50 minutes. Remove cover and bake for an additional 10 minutes.

You can adjust down the amount of corned beef, and/or sauerkraut,  if you wish.  My husband is a huge fan of cheese, so he usually adds more swiss!  This is an awesome recipe to serve on St. Pat's Day!