Preheat the oven to 200°C. Lightly coat the peppers with 1 tablespoon of extra-virgin olive oil and place them on a baking tray. Roast for 30-40 minutes, turning halfway through, until the skins are blistered and the flesh is tender.
Remove the roasted peppers and transfer them to a plastic bag or cover with a bowl for 10-15 minutes. This traps the steam and loosens the skins. Peel the skins off, discard the seeds and stalks, then slice the peppers into strips. Toss the strips with the chopped garlic, chopped parsley and 2 tablespoons of extra-virgin olive oil, then season with sea salt and freshly ground black pepper to taste.
Pat the tuna belly steaks dry with kitchen paper and season both sides with coarse sea salt and freshly ground black pepper.
Heat a heavy-based frying pan or griddle pan over high heat. Add the remaining 2 tablespoons of extra-virgin olive oil. Once the oil is smoking hot, add the tuna belly steaks and cook for 2-3 minutes on each side for medium-rare, or a little longer if you prefer them more well done.
Serve the tuna belly immediately with the roasted peppers and, if you like, drizzle over a little extra-virgin olive oil.