1Heat oven to 350ºF. Grease or spray 2 1/2-qt baking dish.
2Heat water and butter to rapid boil in 3-quart saucepan; remove from heat. Stir in milk and mustard. Stir in 2 pouches potatoes just until moistened. Let stand about 1 minute or until liquid is absorbed. Whip with fork until smooth.
3Spread about 3 cups of the potatoes in baking dish. Top with corned beef. Spread sauerkraut over corned beef and sprinkle with 2 teaspoons caraway seed, if desired. Spoon remaining potatoes over top; spread gently. Sprinkle potatoes with cheese and remaining caraway seed.
4Bake uncovered about 20 minutes or until cheese is light golden brown.