Drain the canned tuna and place it in a mixing bowl.
Add mayonnaise and Dijon mustard to the tuna.
Mix well until the ingredients are combined.
Fold in the chopped celery and red onion.
Season with salt and pepper to taste.
Prepare the bread slices on a clean surface.
Spread a layer of the tuna mixture onto one slice of bread.
Add a layer of leafy greens on top of the tuna.
Top with another slice of bread.
Cut the sandwich in half and serve immediately.