01 - Combine drained tuna, mayonnaise, Dijon mustard, celery, and red onion in a mixing bowl. Mix thoroughly until well incorporated. Season with salt and pepper to taste.
02 - Spread softened butter evenly on one side of each bread slice. This buttered side will face outward during grilling.
03 - Layer half the tuna salad on the unbuttered side of two bread slices, spreading evenly. Add tomato slices if desired, then top each with 2 cheese slices. Cover with remaining bread, buttered side facing out.
04 - Heat skillet over medium heat. Place sandwiches in pan and cook 3-4 minutes per side, pressing gently with spatula, until bread turns golden brown and cheese melts completely.
05 - Remove from heat and slice each sandwich diagonally in half. Serve immediately while hot and cheese remains melted.