1. From lemons, grate 1 teaspoon peel and squeeze 2/3 cup juice. In 9-inch square baking dish, with wire whisk, whisk lemon peel and juice, 3 tablespoons oil, 5 tablespoons parsley, salt, and pepper until mixed. Add tuna, turning to coat. Cover and refrigerate 45 minutes to marinate, turning occasionally.
2. In 10-inch cast-iron or other heavy skillet, heat remaining 3 tablespoons oil over medium-high heat until hot. Add tuna; cook until pale pink in center (medium), about 3 minutes per side or until desired doneness. Transfer to 4 dinner plates and sprinkle with remaining 1 tablespoon parsley.