In a bowl mix together tuna, mayo, celery, onion, lemon juice and salt and pepper. Mix until well combined.
7 oz can tuna, 1/3 cup mayo, 2 Tablespoons chopped celery, 2 Tablespoons finely chopped sweet onion, 1 Tablespoon lemon juice, Salt & Pepper
Butter one side of each slice of bread. Divide the tuna mixture in half, spread onto the unbuttered side, of 2 of the slices of bread. Top with sliced tomato and cheddar cheese. Place the 2nd slice of bread, buttered side out onto the tuna.
Preheat a griddle or large skillet over medium high heat. Place the sandwiches, buttered side down into a hot skillet. Cook for 3-4 minutes until lightly browned. Flip and cook for an additional 3-4 minutes until browned and the cheese is melted.