Open and thoroughly drain the canned tuna. Use a fork or spoon to press out as much liquid as possible. Transfer the drained tuna to a medium mixing bowl.

Add the mayonnaise, finely diced celery, minced red onion, Dijon mustard, salt, and black pepper to the bowl with the tuna. Mix well with a fork until all ingredients are evenly combined.

Evenly spread softened butter on one side of each slice of bread. If using olive oil spray, lightly mist one side of each slice.

Place two slices of bread, butter-side down, in your chosen cooking vessel (air fryer, skillet, or toaster oven).

Divide the tuna salad evenly between the two bread slices, spreading it to the edges. Top each tuna-covered slice with two slices of cheese.

Place the remaining two slices of bread on top of the cheese, butter-side up.

Cook using your preferred method:

* **Air Fryer:** Cook at 350°F (175°C) for 6-10 minutes, flipping halfway through, until golden brown and cheese is melted.

* **Skillet:** Cook over medium heat for 4-6 minutes per side, until bread is golden brown and cheese is bubbly and melted.

* **Toaster Oven (Broil Setting):** Toast on the bottom rack for 3-5 minutes until the bottom is golden, then broil on the top rack for 2-4 minutes until the cheese is melted and bubbly and the top bread is lightly toasted.

Carefully remove the sandwiches from the heat. Let them cool for a minute or two before slicing in half diagonally. Serve immediately.