In a medium bowl, combine the drained tuna with mayonnaise, Dijon mustard, chopped celery, and diced red onion. Stir in the lemon juice, and season with salt and pepper to taste. Mix until all ingredients are fully coated and the mixture is smooth and creamy.

Lay out eight slices of whole-grain bread on a clean surface. Evenly spread softened butter on one side of each slice.

Take a large skillet or griddle and heat it over medium heat until hot enough to sizzle a drop of butter.

Place four slices of the prepared bread, buttered side down, onto the skillet. On each slice, layer one slice of cheddar cheese, followed by a generous scoop of the creamy tuna mixture, a couple of tomato slices, and a handful of fresh arugula or spinach. Top with another slice of cheddar cheese.

Gently place the remaining slices of bread on top of each prepared sandwich, ensuring that the buttered side faces up.

Grill the sandwiches for about 3–4 minutes per side, or until the bread is golden brown and the cheese has melted completely.

Once the sandwiches are perfectly toasted, remove them from the skillet and let them cool for a minute. Cut each sandwich in half on the diagonal to reveal the melty layers inside. Serve warm.