In a medium bowl, combine the drained tuna, mayonnaise, Dijon mustard, chopped celery, and diced red onion. Mix well until creamy and add lemon juice, salt, and pepper.

Lay the slices of whole-grain bread on a clean surface and spread softened butter generously on one side of each slice.

Place a large skillet or griddle over medium heat and allow it to warm.

Carefully place four slices of prepared bread, buttered side down, onto the skillet. Layer a slice of creamy cheddar cheese on each slice.

Spoon a generous scoop of the creamy tuna mixture onto each cheesy slice, followed by tomato slices and arugula or spinach.

Finish off the assembly with another slice of cheddar cheese on top, followed by the remaining bread slices, buttered side up.

Grill the sandwiches for about 3–4 minutes per side, flipping halfway through.

Remove from the skillet, let cool for a minute, and slice in half before serving.