Drain the canned tuna and place it in a mixing bowl.
Add mayonnaise and Dijon mustard to the bowl.
Mix the tuna with a fork until well combined.
Stir in the chopped celery and green onion.
Add salt and pepper to taste, mixing thoroughly.
Toast the bread if desired for extra crunch.
Spread the tuna mixture evenly on one slice of bread.
Layer the lettuce and tomato slices over the tuna.
Top with the second slice of bread.
Cut the sandwich in half and serve immediately.