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