Drain 1 can of tuna
Mix tuna with mayonnaise, salt, and pepper
Add chopped celery or onion if desired
Butter one side of 2 slices of bread
Place bread butter-side down in a skillet
Add the tuna mixture to one slice
Top with cheese
Close the sandwich with the second slice of bread, butter-side up
Cook over medium heat until golden brown
Flip and cook the other side until golden brown and the cheese melts
Remove from skillet and serve hot