In a bowl, combine the drained tuna, mayonnaise, and Dijon mustard.
Add the chopped celery and onion to the mixture.
Season with salt and pepper to taste; mix well.
Lay out the slices of bread on a clean surface.
Spread the tuna mixture evenly on one slice of bread.
Top with lettuce leaves and tomato slices.
If desired, add pickles or olives.
Place the second slice of bread on top.
Gently press down to secure the sandwich.
Cut the sandwich in half and serve immediately.