Let's cook.
In a medium bowl, combine the drained tuna, mayonnaise, finely chopped celery, Dijon mustard, and black pepper. Mix well until all ingredients are thoroughly combined.
Spread butter evenly on one side of each slice of bread.
Heat a large skillet over medium heat. Place two slices of bread, butter-side down, in the skillet.
Spoon half of the tuna mixture onto each of the bread slices in the skillet. Top each with one slice of cheddar cheese.
Place the remaining two slices of bread, butter-side up, on top of the cheese.
Cook for 3-4 minutes per side, or until the bread is golden brown and the cheese is melted and bubbly. Adjust heat as needed to prevent burning.
Remove from skillet, slice in half, and serve immediately.
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