Preheat oven to 250F.

Arrange rye bread cubes in a single layer on a baking sheet.

Toast bread cubes in the oven until dry.

Crush or process toasted bread cubes into medium crumbs.

Set bread crumbs aside.

Increase oven temperature to 350F.

In a large bowl, mix cream of mushroom soup, milk, onion, corned beef, dry mustard, and sugar.

Set soup mixture aside.

Spread sauerkraut evenly in the bottom of a lightly greased 9x13-inch baking dish.

Place potato slices in a single layer, slightly overlapping, over the sauerkraut.

Spoon soup mixture over the potatoes.

Top with shredded Swiss cheese.

In a small bowl, combine melted butter with rye bread crumbs.

Sprinkle bread crumb mixture over the cheese.

Cover the baking dish and bake for 50 minutes.

Remove cover and bake for an additional 10 minutes.