01 -  In a medium mixing bowl, blend tuna, mayonnaise, Dijon mustard, red onion, celery, lemon juice, pepper, and salt. Mix thoroughly until combined. 
 02 -  Arrange four slices of bread on a cutting board. Evenly spread the tuna salad over two slices and layer each with two slices of cheddar cheese. 
 03 -  Top with the remaining bread slices to close the sandwiches. 
 04 -  Spread a thin, even layer of softened butter on the outside surfaces of each slice of bread. 
 05 -  Preheat a large skillet or griddle over medium heat. Carefully transfer the sandwiches, placing them buttered side down into the pan. 
 06 -  Cook each side for 3–4 minutes, pressing gently with a spatula, until the exterior turns golden brown and the cheese melts inside. 
 07 -  Remove sandwiches from the skillet, rest briefly, then slice in half and serve while warm.