Preheat a countertop griddle or stovetop skillet to 350 degrees, or medium-high heat.
Mix the tuna salad ingredients in a large bowl. Set aside.
Spread butter on one side of the bread and set it face down, onto the hot griddle.
Place sliced cheese on the bread and top with about 1/3 cup of tuna salad.
Butter one side of another slice of bread. Place the buttered side up on top of the sandwich.
Cook for about 2 minutes on one side before flipping the sandwich to cook on the other side for another 2 minutes. The tuna melt is done with the cheese is melted and the tuna has warmed through.