Heat 4 tablespoons oil in a frying pan, add the onion, garlic and peppers and cook over a low heat for 10 minutes until soft and golden. Add the chilli flakes and tomatoes, season to taste, cover and leave to simmer for 10 minutes. Uncover and cook for 5 minutes more until very soft and most of the liquid has boiled away.
Meanwhile, rub the remaining oil over the tuna steaks and season thoroughly. Heat a griddle pan until smoking and cook the tuna steaks for 2–3 minutes on each side until charred but still pink inside. Serve the tuna with the peperonata and a rocket salad.