In a small dry skillet, roast the unpeeled garlic and the chiles over medium heat, turning occasionally, until soft (the skins of both will have blackened in spots, which is okay as long as the flesh doesn't burn), 5 to 10 minutes for the chiles, 15 minutes for the garlic. Cool, then peel the garlic. Place both garlic and chiles in a food processor or blender along with the lime juice. Run the machine until the mixture is as smoothly pureed as possible. Season highly with salt, usually about 3/4 teaspoon.
Scoop two-thirds of the marinade into a large non aluminum baking dish. Lay the fish in the marinade and smear the mixture on all sides of each piece. Cover and refrigerate while you prepare the salsa.
Scrape the remaining marinade into a medium bowl. Mix in the chopped tomatoes. Scoop the onion into a small strainer, rinse under cold water and shake off the excess; and add to the tomatoes. Stir in the herbs. Taste and season with additional salt, usually about 1/2 teaspoon.
Heat a gas grill to medium-high or light a charcoal fire and let it burn until the coals are covered with gray ash and very hot (the fire feels almost intolerably hot to most cooks when they hold there hand 4 or 5 inches above the grill grates for 5 seconds). Set the cooking grate in place, cover the grill and let the grate heat up, 5 minutes or so. Remove the fish from the marinade, brush or spray with a generous coating of oil and lay on the hot grill. Cover the grill and cook 3 minutes. Uncover, flip the fish over, cover once again and cook until the fish is as done as you like - 1 to 2 minutes more is all that's needed for medium-rare tuna (my preference).
Transfer the fish to dinner plates, spoon on the beautiful salsa and decorate with herb sprigs - and you're ready to eat.
Working Ahead. There is little in this recipe that can be done in advance - except the marinade, which will hold for a day or so in the refrigerator if well covered. If the fish spends more than three or four hours in that limey marinade, it will get pickled. Beautiful tomatoes with salt and lime on them also begin to wilt after an hour or two.