In a medium bowl, combine the sauerkraut and corned beef; mix well.
In another medium bowl, combine the soup, dressing, milk, onion and mustard; mix well then spread 1/2 cup of the mixture over the bottom of the baking dish.
Place three lasagna noodles over the soup mixture. Top with half of the corned beef mixture and then half of the remaining soup mixture. Layer with three more lasagna noodles then the remaining corned beef mixture. Add the remaining three lasagna noodles then cover with the remaining soup mixture. Sprinkle the Swiss cheese then bread crumbs over the top and drizzle with the butter.
Cover with aluminum foil and bake for 45 to 50 minutes, or until bubbly. Uncover and bake for 5 to 10 more minutes, or until golden. Allow to sit for 5 to 10 minutes, then cut and serve.
Makes 6 to 8 servings