Homemade salsa is a wonderful way to highlight heirloom tomatoes. In the United States, people refer to this kind of salsa as "pico de gallo." I'm adding it along with some surprising herbs to a grilled fillet of tuna, creating a refreshing meal you'll want to enjoy again and again in the summer.
Marinade: Preheat a gas or charcoal grill to medium-high heat. Grill the chiles and unpeeled garlic cloves, turning occasionally, until they are soft and the skins have blackened in spots, about 15 minutes. Remove from heat and cool.Peel the garlic; add to blender. Coarsely chop chiles, then add to blender along with the lime juice. Starting on low and ending on high speed, blend until the mixture is as smoothly puréed as possible.
Place tuna steaks into a large baking dish and pour ⅔ of the purée on top (reserving the remainder for the salsa); spread it evenly over the fish with a spatula. Marinate in the refrigerator for up to 1 hour.
While the fish is marinating, prepare the Heirloom Tomato Salsa: Finely chop the tomatoes, and combine with the remaining marinade in a medium bowl. Destem the fresh herbs, finely chop the leaves, and add to the tomatoes (about ¼ cup total). Finely dice the red onion, place in a fine-mesh strainer and rinse under cold running water. Shake off the excess water and add to the tomatoes. Stir to combine; add salt to taste, and stir again.
Brush tuna steaks with a generous coating of oil. Season with salt, and place on the hot grill. Cover the grill and cook until medium-rare, 4–5 minutes for the first side. Turn and cook for another minute before removing from heat.
Transfer the fish to a serving platter. Spoon the Heirloom Tomato Salsa over the fish and garnish with herb sprigs. Serve immediately with additional Heirloom Tomato Salsa on the side.