01 - Lay out the rye bread slices on a clean cutting board.
02 - Spread mustard evenly on one side of each bread slice.
03 - Layer half the corned beef onto two of the bread slices.
04 - Top the corned beef with Swiss cheese (if using) and pickle slices.
05 - Close each sandwich with the remaining bread slices, mustard side down.
06 - Melt butter in a skillet over medium heat. Grill sandwiches on each side for 2-3 minutes until golden and heated through.
07 - Slice sandwiches in half and serve immediately.