Spread Dijon mustard to taste over slices of bread. Build sandwiches with two slices of corned beef topped with sauerkraut.
Mix Swiss and Cheddar cheeses together; divide between sandwiches. Season with pepper and close sandwiches. Press top slice down with spatula.
Spread top slice of each sandwich with butter. Place remaining butter in large frying pan and melt over medium heat.
When hot, set sandwiches in pan, buttered-side-up, and cook 2 minutes. Turn sandwiches over and continue cooking 2 minutes or long enough to melt cheese.
Slice and serve hot.