Butter one side of each slice of bread.
Place two slices of bread, butter side down, on a skillet or griddle over medium heat.
Divide the tuna mixture evenly between the two slices of bread on the skillet.
Place a slice of cheese on top of the tuna mixture on each sandwich.
Top with the remaining slices of bread, butter side up.
Cook until the bottom slice of bread is golden brown, about 3-5 minutes.
Flip the sandwiches and cook until the other side is golden brown and the cheese is melted, about 3-5 minutes more.
Remove from heat, cut in half, and serve immediately.