Heat oven to 350ºF. Grease square baking dish, 8x8x2 inches.
Heat hot water, milk and margarine to rapid boil in 3-quart saucepan; remove from heat.
Stir in 2 pouches Potatoes and Seasoning just until moistened. Let stand about 1 minute or until liquid is absorbed. Make sure the mixture is cool, or the mustard will curdle. Stir in mustard. Whip with fork until smooth.
Spread 1 1/2 cups of the potatoes in baking dish.
Top with corned beef.
Spread sauerkraut over corned beef.
Spoon remaining potatoes over top; spread gently.
Sprinkle potatoes with cheese and caraway seed.
Bake uncovered about 20 minutes or until cheese is golden brown.