Prepare the Tuna Salad:
In a mixing bowl, combine the drained tuna, mayonnaise, Dijon mustard, lemon juice, and chopped onion or celery if using. Mix well and season with salt and pepper to taste.
Assemble the Sandwich:
Spread an even layer of tuna salad on two slices of bread. Top each with a slice of cheese, then place the remaining bread slices on top to form two sandwiches.
Butter the Bread:
Spread the softened butter on the outer sides of the bread slices.
Cook the Sandwich:
Skillet Method: Heat a skillet over medium heat. Place the sandwiches in the skillet and cook for 3–4 minutes per side, pressing lightly with a spatula, until the bread is golden and the cheese is melted.
Broiler Method: Preheat the broiler. Place the sandwiches on a baking sheet and broil for 2–3 minutes on each side, keeping a close eye to prevent burning.
Serve:
Slice the sandwiches in half and serve hot, optionally with a side of pickles, chips, or a salad.