01 - Arrange bread slices on a clean surface.
02 - Spread mayonnaise evenly over each slice; optionally, spread Dijon mustard on two slices.
03 - Place sliced turkey breast evenly on two bread slices.
04 - Top turkey with lettuce, tomato, cucumber, and red onion if using.
05 - Sprinkle salt and freshly ground black pepper to taste.
06 - Close each sandwich with remaining bread slices, mayonnaise side down.
07 - Cut sandwiches in half and serve immediately.