Drain 1 can of tuna
Mix tuna with mayonnaise
Add a little salt and pepper
Add chopped celery or onion if desired
Butter one side of each bread slice
Place one slice of bread butter-side down in a skillet
Add a slice of cheese
Add the tuna mixture on top of the cheese
Add another slice of cheese on top of the tuna
Cover with the second slice of bread, butter-side up
Cook over medium heat until the bread is golden brown
Flip carefully and cook the other side until golden brown and the cheese is melted
Remove from the skillet and slice in half
Serve warm