Step 1: Prepare Tuna Salad : In a medium bowl, combine drained tuna, mayonnaise, Dijon mustard, celery, red onion, and lemon juice. Season with salt and pepper to taste. Mix until well combined.

Step 2: Assemble Sandwiches : Place a generous scoop of tuna salad on 4 slices of bread. Top each with cheddar cheese. Place the remaining bread slices on top to form sandwiches.

Step 3: Grill the Sandwiches : Heat a skillet or griddle over medium heat. Spread butter on the outside of each sandwich. Grill sandwiches 3–4 minutes per side, or until bread is golden brown and cheese is melted. Press gently with a spatula to ensure even grilling.

Step 4: Serve : Cut sandwiches in half diagonally. Serve warm with pickles, potato chips, or a side salad.