Spread 1 side of each bread slice with mustard. Cut into cubes. You should have about 6 cups (1.5 L) bread cubes.
Layer bread cubes, corned beef, sauerkraut and cheese, in order given, in greased 9 x 13 inch (22 x 33 cm) pan.
Beat remaining 4 ingredients in medium bowl until well combined. Pour over cheese. Cover with foil. Chill overnight. Bake, covered, in 360 F (175°C) oven for 40 minutes. Remove foil. Bake for about 40 minutes until bubbling and cheese is golden brown.