In a mixing bowl, combine the drained tuna, mayonnaise, Dijon mustard, red onion, lemon juice, and a pinch of salt and pepper. Mix until well combined.

Preheat a skillet over medium heat.

Butter one side of each bread slice.

Place two slices of bread, butter side down, on the skillet.

Spoon half of the tuna mixture on top of each bread slice.

Add the remaining slices of bread on top, butter side up.

Cook until the bread is golden brown and the cheese is melted, about 3-4 minutes per side.

Serve hot and enjoy your delicious tuna melt!