Spread 4 of the bread slices with mustard;top with corned beef, coleslaw and cheese. Top with remaining slices of bread. Melt butter over medium-low heat on a griddle or in a very large skillet. Add sandwiches; cook 3 to 4 minutes per side or until bread is toasted and sandwich is heated through. Great served with chips or deli potato salad and accompanied with a dill pickle.