1. Line the basket of an air fryer with parchment paper.
2. In a bowl, combine the tuna, celery, mayonnaise, onion, gherkins, parsley and lemon juice. Season with salt and pepper.
3. Lay the bread slices out on a work surface. Top two slices of bread each with a slice of cheese. Garnish with the tuna mixture and tomato slices. Top with the remaining cheese. Close the sandwiches with the remaining slices of bread. Cover the outside of the sandwiches with the butter.
4. Place one sandwich at a time in the basket of the air fryer. Cook for 10 minutes at 400°F (200°C) or until the bread is golden and the cheese is melted, flipping the sandwich over halfway through.
5. Cut the sandwiches in half. Serve with more gherkins and green salad, if desired.

Chef’s tip

For an even crispier sandwich, cover the outside with mayonnaise instead of butter before cooking. The bread will brown beautifully in the air fryer.