Preheat oven to 200 degrees F. Using a sharp knife, make 9 slits in the tuna filet. Fill each slit with 1 clove, 1 coriander seed and 1 garlic slice. Season tuna with salt and pepper. In an oven-proof pot, heat oil over medium heat. Add onion and saute about 8 minutes. Push to the side of the pot. Add tuna and saute about 12 minutes, until brown on all sides. Add remaining garlic, the oregano and tomatoes to the pot, scattering around the tuna. Season with additional salt and pepper. Pour in vinegar over the tuna. Bake uncovered in oven until a thermometer inserted in tuna reaches 145 degrees F (medium rare), or about 12 minutes. Transfer fish to a serving platter and spoon the tomato mixture over it. Let cool to room temperature. Slice tuna and serve. Serves 6.