Open the can of tuna and drain the liquid.
In a mixing bowl, combine the tuna, mayonnaise, and Dijon mustard.
Add the finely chopped celery and red onion to the bowl.
Season the mixture with salt and pepper to taste.
Mix all ingredients until well combined.
Toast the bread slices until golden brown (optional).
Spread the tuna mixture evenly on one slice of bread.
Add lettuce leaves and sliced tomatoes if desired.
Top with the second slice of bread.
Cut the sandwich in half and serve immediately.