Preparation

• In a medium bowl, combine the drained tuna, mayonnaise, Dijon mustard, finely chopped celery, and diced red onion. Add lemon juice, salt, and pepper to taste. Mix well until all ingredients are creamy and evenly incorporated.
• Lay out the eight slices of whole-grain bread, spread a generous layer of softened butter on one side of each slice.
• Place a large skillet over medium heat for about 2 minutes until warm.
• Place four slices of the prepared bread, buttered side down, directly onto the skillet. Cook for about 1-2 minutes until the edges begin to turn golden.
• Top each slice in the skillet with a slice of cheddar cheese, a scoop of the tuna mixture, fresh tomato slices, and a handful of arugula or spinach, then finish with another slice of cheddar.
• Place the remaining slices of bread on top of the sandwiches, buttered side up, and gently press down.
• Grill the sandwiches for about 3-4 minutes on one side, then gently flip and grill the other side for another 3-4 minutes until both sides are crispy and cheese is melted.
• Remove the sandwiches and let them cool for a minute, then slice in half diagonally and serve warm.