In a medium bowl combine the soup, milk, onion and mustard. Mix well and set aside.
Spread sauerkraut into the bottom of a lightly greased 20x30cm baking dish. Top with uncooked noodles, then spoon soup mixture evenly over noodles. Cover all with kielbasa and top with cheese. Lastly, combine bread crumbs and melted butter in a small bowl and mix together; sprinkle mixture over cheese.
Cover baking dish tightly with aluminium foil and bake in the preheated oven for 1 hour, or until noodles are tender.