Preheat oven to 350 degrees
Dice onion and mix with sour cream set aside for later

Take 8 slices of rye bread and cube. Place a layer in bottom of 9 x 13 baking dish, put leftover bread pieces to side for top.
Melt 1/2 stick of butter and pour over bread in bottom of pan.
Drain sauerkraut and layer on top of bread.
Place sliced corned beef on top of sauerkraut.
Spread onion and sour cream mixture on top of corned beef.
Layer shredded swiss cheese next.
Top with leftover rye bread (you may need to cut more).
Melt other 1/2 stick butter and pour over top.

Bake at 350 degrees until cheese boubbles and bread begins to toast.