Finely chop the onion and bell pepper.
Heat olive oil in a pan and sauté the onion and bell pepper.
Add spicy tuna (after removing excess oil) and cook together.
Season with salt and pepper.
Transfer the sautéed ingredients to an oven-safe dish.
Generously top with mozzarella cheese.
Bake in the oven or air fryer at 200°C (392°F) for 10 minutes.