Preheat oven to 350 degrees.
Cook noodles according to package directions. Meanwhile, in a small bowl, combine the soup, milk, onion and mustard; set aside.
Spread sauerkraut into a 1-qt. baking dish coated with cooking spray. Drain noodles; place over sauerkraut. Layer with soup mixture and kielbasa; sprinkle with cheese.
In a small bowl, combine bread crumbs and butter; sprinkle over casserole. Bake, uncovered, for 15–20 minutes or until bubbly.