In a bowl, combine drained tuna, mayonnaise, Dijon mustard, celery, onion, salt, and pepper.
Mix well until all ingredients are fully incorporated.
Butter one side of each slice of bread generously.
Preheat a skillet over medium heat.
Place one slice of bread, butter side down, in the skillet.
Spread the tuna mixture evenly over the slice in the skillet.
Sprinkle shredded cheese on top of the tuna mixture.
Top with the second slice of bread, butter side up.
Cook for 3-4 minutes until the bottom is golden brown.
Carefully flip the sandwich and cook for another 3-4 minutes until the cheese melts and the other side is golden.