01 - Lay out the rye bread slices on a clean cutting board or flat surface.
02 - Spread mustard evenly on one side of each bread slice using a spreading knife.
03 - Distribute half the corned beef onto two of the bread slices, creating an even layer.
04 - Top the corned beef with Swiss cheese slices (if using) and arrange 4-6 pickle slices on each sandwich.
05 - Close each sandwich with the remaining bread slices, placing the mustard-coated side facing inward.
06 - For a warm, toasted sandwich, melt butter in a skillet over medium heat. Grill sandwiches on each side for 2-3 minutes until golden brown and heated through.
07 - Cut each sandwich diagonally in half with a bread knife and serve immediately while warm.