Remarkably lean for so tasty a fish, tuna provides good amounts of healthy omega-3 fatty acids. It is also an excellent source of B vitamins. Swordfish has even less fat than tuna, with a good supply of niacin.
Preheat the oven to 180°C. Combine the capsicums, chilli, vinegar, coriander, mint, onion, lime juice, 2 tablespoons of the oil, and salt and pepper to taste, in a medium bowl. Put aside for 30 minutes to allow the flavours to develop.
Heat the remaining oil on a griller tray or in a large nonstick frying pan over moderately high heat. Cook the tuna for 2 to 3 minutes each side or until rare to medium. Top with a little more pepper, then serve with the salsa and the lemon wedges alongside.