In a mixing bowl, combine the drained tuna, mayonnaise, Dijon mustard, chopped celery, chopped red onion, garlic powder, salt, and pepper. Mix until well combined.
Preheat the skillet or griddle over medium heat.
Take a slice of bread and spread a thin layer of softened butter on one side. Place the buttered side down on a clean surface.
Add a generous amount of the tuna mixture on top of the unbuttered side of the bread.
Sprinkle a portion of the shredded cheddar cheese on top of the tuna mixture.
Take another slice of bread, butter one side, and place it on top of the cheese, buttered side up.
Carefully place the sandwich on the preheated skillet. Cook for about 4-5 minutes or until the bottom is golden brown and crispy.
Carefully flip the sandwich over and cook for an additional 3-4 minutes, or until the other side is golden brown and the cheese is melted.
Remove the sandwich from the skillet, let it cool for a minute, then slice in half.
Repeat the process for the remaining sandwiches.