Lay out the ingredients on a clean surface.
Toast the slices of rye bread lightly for extra crunch.
Spread mustard generously on one side of each slice of bread.
Place the Swiss cheese on one slice of bread.
Layer the pastrami evenly over the cheese.
Add the lettuce and tomato slices on top of the pastrami.
If desired, place pickles on the top layer.
Close the sandwich with the second slice of bread, mustard side down.
Press down gently to secure the layers.
Cut the sandwich in half diagonally and serve immediately.