Brush the tuna with some oil and sprinkle with salt and pepper. Heat the grill and brush the cooking grates with oil.
Heat the oil in a small skillet over medium heat and cook the garlic for a minute. Add the tomatoes and cook for another couple of minutes. Add the wine and bring to a rapid boil for two minutes. Add all remaining ingredients except for the parsley and simmer uncovered for 5 minutes. Stir in the parsley and remove from heat.
Sear the tuna steaks over high, direct heat for 1-2 minutes on each side. (For the best texture and to prevent becoming dried out tuna should be rare to medium rare on the inside.)
Serve the tuna steaks immediately topped with the sauce.