Gather all ingredients on a clean surface.
Lay one slice of bread on a cutting board.
Spread mayonnaise or horseradish sauce on the slice.
Add a layer of roast beef evenly across the bread.
Sprinkle salt and pepper over the beef.
Place the cheese slice on top of the beef if using.
Add the lettuce leaf on top of the cheese.
Layer the tomato slices over the lettuce.
Top with the second slice of bread.
Cut the sandwich in half for easier handling.