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