In a mixing bowl, combine the drained tuna, mayonnaise, Dijon mustard, diced celery, and diced onion.
Season the mixture with salt and pepper to taste.
Spread butter on one side of each slice of bread.
Place one slice of bread, buttered side down, on a skillet over medium heat.
Spread the tuna mixture evenly on top of the bread in the skillet.
Place a slice of cheese on top of the tuna mixture.
Cover with the second slice of bread, buttered 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 brown.
Remove from skillet, slice in half, and serve immediately.