Prepare the tuna marinade, whisking ingredients together, pour into zip-top bag, add tuna and marinate for 20 minutes at room temperature. Combine the tomatoes, sugar and salt, toss - place in a colander, let drain for 10 - 15 minutes. Mix the cooked rice with the garlic, scallions, capers, and red pepper flakes. Whisk together the the 1/4 cup lemon juice and 1/4 cup olive oil. Pour over rice and stir well, set aside. To cook the tuna, drain the marinade into a large skillet that has been heated over medium-high heat. Add the tuna steaks, cook approx. 1 - 2 minutes per side for rare, remove from the pan and let cool for 8 - 10 minutes. Add the tomatoes, parsley and mint to the rice. Stir well, place on a platter, slice tuna. Refrigerate for at least 1 hour. Serve.