Gather all ingredients on a clean surface.
Lay out the slices of rye bread.
Spread mustard or mayonnaise on one slice of bread.
Layer the sliced pastrami evenly over the spread.
Add a slice of Swiss cheese if desired.
Place the lettuce leaves on top of the cheese.
Add the tomato slices over the lettuce.
Top with the second slice of rye bread.
Press down gently to combine the layers.
Slice the sandwich in half and serve immediately.