In a small skillet, roast the unpeeled garlic and chilies over a medium heat, turning occasionally until soft, about 5 to 10 mins for the chilies and 15 mins for the garlic.
Cool and then peel the garlic. Put the chilies and the garlic in a food processor or blender, add the lime juice and blend until mixture is smooth. Season with salt.
Marinate the tuna in about 2/3 of the marinade, making sure the tuna is well covered with the marinade. Cover and refrigerate while you prepare the salsa.
Place the remaining marinade into a medium bowl. Mix in diced tomatoes and onions; stir in fresh herbs and season with about 1/2 a teaspoon of salt.
Heat your gas grill to a medium heat or a charcoal fire until very hot. Cover the grill and let the cooking grate heat up for about 5 minutes.
Remove tuna from marinade, brush with generous coating of olive oil and lay on hot grill. Cover and cook for 3 min, turn tuna over and cover again for another 1 to 2 mins for medium rare or longer to taste.
Transfer tuna to dinner plates and spoon on salsa, garnish with herb sprigs and serve with your favorite Terra chips.