In a mixing bowl, add the drained tuna and flake it with a fork. Add mayonnaise, Dijon mustard, lemon juice, celery, red onion (if using), garlic powder, onion powder, salt, black pepper, and paprika. Mix until everything is well combined and creamy but not mushy.

If using keto bread or chaffles, lightly toast them for better structure. If using portobello mushrooms, wipe them clean and lightly roast or pan-cook to remove excess moisture. For lettuce wraps, wash and dry the leaves thoroughly.

Spread a generous layer of the tuna mixture onto your chosen base. Top with shredded or sliced cheddar cheese and mozzarella if using.

Heat a non-stick skillet over medium-low heat with butter or oil. Place the assembled tuna melt in the skillet, cover with a lid, and cook until the cheese is fully melted and bubbly.

Preheat the oven to 375°F (190°C). Place the tuna melts on a baking sheet and bake for 8–12 minutes, or until the cheese is melted and lightly golden.

Air fry at 350°F (175°C) for 5–7 minutes, checking frequently to avoid over-browning.

Remove from heat and let rest for 1–2 minutes. Garnish with parsley or green onions if desired.