01 - Lay out all four rye bread slices on your work surface. Spread mustard evenly across two slices. If using mayonnaise for a creamier sandwich, spread it on the remaining two slices.
02 - Place Swiss cheese slices on top of the mustard-covered bread slices. If omitting cheese or preferring a dairy-free version, skip this step.
03 - Distribute the sliced corned beef evenly over the cheese layer. For sandwiches without cheese, pile the corned beef directly onto the prepared bread slices.
04 - Arrange dill pickle slices over the corned beef. Add thinly sliced red onion and shredded lettuce if desired for extra crunch and flavor.
05 - Top each sandwich with the remaining bread slice, placing mayonnaise-side down if applicable. Press gently to adhere layers.
06 - Using a sharp bread knife, cut each sandwich diagonally in half. Serve immediately while bread remains fresh and crisp.