Making tuna melt sandwiches is straightforward. Follow these easy steps for a delicious outcome:

1. Prep the Tuna Salad: In a mixing bowl, combine the drained tuna, mayonnaise, Dijon mustard, chopped celery, chopped red onion, salt, and pepper. Mix until well combined.

2. Assemble the Sandwiches: Lay out two slices of bread on a clean surface. Scoop a generous portion of the tuna salad onto each slice.

3. Add Cheese: Place a slice of cheese over the tuna salad on each sandwich. Top with the remaining slices of bread.

4. Butter the Bread: Spread a thin layer of butter on the outer sides of each sandwich.

5. Grill the Sandwiches: Heat a skillet over medium heat. Place the sandwiches in the skillet, cooking until golden brown on the bottom, about 3-5 minutes.

6. Flip and Continue Grilling: Carefully flip the sandwiches over and cook the other side until the cheese has melted and the bread is golden brown, about another 3-5 minutes.

7. Serve: Remove the sandwiches from the skillet and let them cool for a minute. Then, cut them in half diagonally for a beautiful presentation.

These steps will guide you in creating the ultimate tuna melt sandwich that is both delicious and satisfying.