In a bowl, mix tuna, mayonnaise, Dijon mustard, celery, red onion, and lemon juice.
Season with salt and black pepper and stir until combined.
Spread butter on one side of each bread slice.
Place four slices butter-side down in a skillet over medium heat.
Spoon tuna mixture evenly onto each slice.
Top each with a slice of cheddar cheese.
Cover with remaining bread slices, butter-side up.
Cook for 3–4 minutes until golden brown.
Flip carefully and cook another 3–4 minutes until cheese melts.
Remove from skillet and slice in half before serving.
