Preparation Preheat oven to 200°F. Using knife, make 9 slits in sides of tuna. Fill each with 1 clove, 1 coriander seed, and 1 garlic sliver; sprinkle tuna with salt and pepper. Heat oil in ovenproof pot over medium heat. Add onion and sauté 8 minutes; push to side of pot. Add tuna and sauté until brown on all sides, about 12 minutes. Scatter tomatoes, oregano, and remaining garlic slivers around tuna. Sprinkle with salt and pepper. Pour vinegar over fish. Bake tuna uncovered until thermometer inserted into center registers 145°F for medium-rare, about 12 minutes. Transfer tuna to platter. Spoon tomato sauce over. Cool to room temperature. Slice tuna thinly and serve.
Preparation Preheat oven to 200°F. Using knife, make 9 slits in sides of tuna. Fill each with 1 clove, 1 coriander seed, and 1 garlic sliver; sprinkle tuna with salt and pepper. Heat oil in ovenproof pot over medium heat. Add onion and sauté 8 minutes; push to side of pot. Add tuna and sauté until brown on all sides, about 12 minutes. Scatter tomatoes, oregano, and remaining garlic slivers around tuna. Sprinkle with salt and pepper. Pour vinegar over fish. Bake tuna uncovered until thermometer inserted into center registers 145°F for medium-rare, about 12 minutes. Transfer tuna to platter. Spoon tomato sauce over. Cool to room temperature. Slice tuna thinly and serve.