Gather all ingredients: pastrami, bread, lettuce, tomatoes, and mustard.
Slice the tomatoes and wash the lettuce.
Lay two slices of bread on a clean surface.
Spread mustard on one slice of bread.
Layer the pastrami on top of the mustard generously.
Add the lettuce over the pastrami.
Top with sliced tomatoes for added freshness.
Cover with the second slice of bread.
Press down gently to compact the sandwich.
Slice the sandwich in half and serve immediately.