01 - Lay out 4 slices of bread on a clean surface.
02 - Spread mayonnaise evenly over all bread slices. Optionally spread Dijon mustard on two slices.
03 - Layer sliced turkey breast evenly over two of the bread slices.
04 - Top the turkey with lettuce leaves, tomato slices, cucumber slices, 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 each sandwich in half and serve immediately.