Prepare the bread: Spread Dijon mustard on one slice of rye bread and mayonnaise on the other slice, if using.

Layer the sandwich: On the mustard-coated slice, pile the pastrami slices evenly. Top with Swiss cheese.

Toast (optional): If desired, you can toast the sandwich in a skillet over medium heat, pressing it slightly to warm the pastrami and melt the cheese. Cook for 3-4 minutes per side, until golden brown and crispy.

Serve: Once the sandwich is ready, slice it in half and garnish with pickle slices, if desired.