In a mixing bowl, combine the drained tuna, mayonnaise, Dijon mustard, garlic powder, onion powder, salt, and pepper. Mix well.
Butter one side of each slice of bread.
Place 4 slices of bread, butter-side-down, on a skillet or griddle.
Top each with a slice of cheddar cheese, a generous spoonful of tuna mixture, and another slice of cheese.
Place remaining bread slices on top, butter-side-up.
Cook over medium heat until the bread is golden brown and the cheese is melted, about 3-4 minutes per side.
Serve hot and enjoy!