Drain the canned tuna thoroughly.
In a mixing bowl, combine the tuna, mayonnaise, and Dijon mustard.
Add chopped celery and red onion to the bowl.
Mix well until all ingredients are combined.
Season with salt and pepper to taste.
Take one slice of bread and layer it with leafy lettuce.
Spread the tuna mixture generously over the lettuce.
Add slices of tomato on top of the tuna salad.
Cover with the second slice of bread.
Cut the sandwich in half and serve immediately.