Preheat the air fryer: Set it to 375°F (190°C) for a few minutes. This helps the bread crisp quickly and the cheese melt evenly.

Make the tuna salad: In a small bowl, combine drained tuna, mayonnaise, Dijon, onion, celery, lemon juice, and a pinch of salt and pepper. Mix until creamy but not runny. Adjust mayo and seasoning to taste.

Prep the bread: Spread a thin layer of softened butter or mayo on one side of each slice. This side will face outward to crisp in the air fryer.

Assemble the melt: Place one slice of bread buttered-side down. Add a slice of cheese, then a generous layer of tuna salad, then another slice of cheese. Top with the second slice of bread, buttered-side up. Press gently to secure.

Air fry: Place the sandwich in the air fryer basket. Cook at 375°F (190°C) for 4–6 minutes. Carefully flip and cook another 3–5 minutes, until the bread is golden and the cheese is melted.

Check doneness: If the cheese isn’t fully melted but the bread is browned, lower the temp to 350°F (175°C) and cook 1–2 more minutes.

Cool briefly and serve: Let the sandwich rest for 1 minute. Slice and serve with chips, salad, or fruit.