Preheat a skillet over medium heat.
In a bowl, mix the drained tuna, mayonnaise, Dijon mustard, celery, and onion.
Add salt and pepper to taste.
Spread butter on one side of each bread slice.
Place one slice of bread butter-side down onto the skillet.
Spread the tuna mixture evenly on the bread in the skillet.
Top with slices of cheese.
Cover with the second slice of bread, butter-side up.
Cook for 3-4 minutes until golden brown, then flip carefully.
Cook for another 3-4 minutes until the cheese melts and the bread is golden.