Preheat oven to 350 degrees. Grease 13x9 inch baking dish.Combine soup, milk, onion, and mustard in a medium bowl.Spread sauerkraut on bottom of prepared pan, pressing firmly. Add noodles. Spoon soup mixture evenly over noodles; cover with sliced sausage. Top with cheese.Combine bread crumbs and melted butter in a small bowl; sprinkle over cheese.Cover tightly with aluminum foil. Bake 1 hour or until noodles are tender.
You can add a bit more milk to be sure to cover the noodles. You can also increase the mustard to a full tablespoon.