Creating the perfect Tuna Melt on Whole Grain Bread is easy with these simple steps:

1. Prepare the Tuna Mixture: In a medium bowl, combine the drained tuna, mayonnaise, Dijon mustard, chopped red onion, and celery. Mix well until thoroughly combined. Season with salt and pepper to taste.

2. Heat the Pan: Place a skillet over medium heat and add one tablespoon of butter, allowing it to melt and coat the surface.

3. Assemble the Sandwich: Spread a generous portion of the tuna mixture onto two slices of whole grain bread. Sprinkle shredded cheddar cheese over the tuna.

4. Top It Off: Place the remaining two slices of bread on top to close the sandwiches.

5. Cook One Side: Carefully place the sandwiches in the skillet. Cook for about 3-4 minutes until golden brown.

6. Flip: Once the first side is toasted, add another tablespoon of butter to the skillet. Carefully flip both sandwiches and cook the other side for another 3-4 minutes until golden brown and the cheese is melted.

7. Check for Doneness: Ensure the cheese is bubbling and melted inside. If necessary, lower the heat and cook for an additional minute.

8. Slice and Serve: Once done, remove the sandwiches from the skillet and let them cool slightly. Slice each sandwich in half diagonally.

9. Optional Add-Ons: Add lettuce, tomatoes, or pickles to the inside of the sandwich, if desired.

10. Enjoy: Serve your Tuna Melt with a side of chips, a salad, or on its own to savor every bite.