Combine the drained tuna, mayonnaise, Dijon mustard, red onion, lemon juice, salt, and pepper in a mixing bowl. Stir well until thoroughly mixed.
Heat a skillet over medium heat before use.
Spread butter on one side of each bread slice.
Take two slices of bread and place them, butter side down, on the heated skillet.
Spoon half of the tuna mixture onto each bread slice.
Layer sliced tomatoes on top of each bread slice.
Sprinkle half of the grated cheddar cheese on each slice.
Place the remaining slices of bread on top, with the butter side facing up.
Cook until the bread turns golden brown and the cheese melts, approximately 3-4 minutes per side.
Serve the delicious tuna melt while hot and enjoy!