01 - Lay out the bread slices on a clean surface.
02 - Spread mayonnaise evenly over all slices. If desired, spread Dijon mustard on two of the slices.
03 - Layer sliced turkey breast on two of the bread slices.
04 - Top each sandwich half with lettuce, tomato, cucumber, and red onion if using.
05 - Sprinkle a pinch of salt and freshly ground black pepper over the vegetables.
06 - Close sandwiches with the remaining bread slices, mayonnaise side down.
07 - Slice each sandwich in half and serve immediately.