Spread mustard (if using) on 1 slice of bread and mayonnaise or butter on the other slice. Tip: butter is a good alternative for people (kids especially) who don’t like mayonnaise.
Onto one slice of bread, spread side-up, layer on the turkey, cheese, thinly sliced apple, and lettuce. Top with second slice of bread, spread side-down.
Cut sandwich in half or quarters. Serve immediately or wrap for a future meal.