In a medium mixing bowl, combine the well-drained tuna, mayonnaise, finely diced celery, minced red onion, Dijon mustard, fresh lemon juice (if using), salt, and black pepper. Mix gently until all ingredients are well combined. Taste the tuna salad and adjust seasoning as needed.

Lay out the four slices of bread on a clean surface. Generously butter one side of each slice. Flip two of the slices over so the unbuttered side is facing up.

Divide the prepared tuna salad mixture evenly between the two unbuttered bread slices. Top each with half of the sharp cheddar cheese (sliced or shredded). Place the remaining two buttered bread slices on top, ensuring the buttered side is facing up, to complete the sandwiches.

Heat a large frying pan or griddle over medium-low heat. Once hot, carefully place the assembled sandwiches into the pan, buttered side down. Cook for 4-8 minutes per side, or until the bread is golden brown and crispy, and the cheese is thoroughly melted and bubbly. You may need to gently press down on the sandwiches with a spatula to ensure even contact and browning. Adjust heat if necessary to prevent burning while ensuring the cheese melts.

Once cooked to your desired crispness and meltiness, remove the tuna melts from the pan. Let them rest for a minute to cool slightly before slicing in half diagonally.

Serve the Ultimate Crispy Tuna Melt Sandwiches immediately with your favorite side, such as a pickle spear, potato chips, or a simple green salad.