01 - In a mixing bowl, combine drained tuna, mayonnaise, Dijon mustard, diced celery, and red onion. Mix thoroughly until evenly distributed. 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 when cooking to create a golden, crispy exterior.
03 - On the unbuttered side of two bread slices, spread half the tuna salad mixture evenly. Layer with tomato slices if desired, then top each with 2 cheese slices. Place remaining bread slices on top, buttered side facing out.
04 - Heat skillet over medium heat. Place sandwiches in the pan and cook for 3-4 minutes per side, pressing gently with spatula. Flip carefully and continue cooking until bread is golden brown and cheese is fully melted.
05 - Remove from heat and slice each sandwich diagonally in half. Serve immediately while hot and cheese is still melted.