Mix together the garlic, lime rind and half the juice, 2 tbsp of the oil and 1 tbsp of rosemary in a large dish. Add the tuna and coat with the marinade - marinate for 30 minutes.
Preheat oven to gas 7, 220ºC, fan 200ºC. Place the tomatoes and onion in a shallow oven proof dish with the remaining rosemary. Drizzle with the remaining oil and season. Roast in the oven for 15 to 20 minutes, until tender and lightly browned.
Lightly oil a ridged cast iron grill pan or large frying pan. Heat the pan and cook the tuna steaks for 5 to 6 minutes turning once. Serve with the tomatoes and onion and drizzle the remaining lime juice over the top.