Preheat the oven to 400 degrees F.
Place the fish in a large colander and rinse under cold running water. Place in a large non-reactive baking dish, and make 5 horizontal 3-inch slits on each side. Season the fish on both sides with 2 tablespoons olive oil, 3 teaspoons kosher salt, and 1/2 teaspoon black pepper.
In a mixing bowl, toss together the sliced tomatoes, onions, garlic, remaining 1 tablespoon olive oil, 1 teaspoon kosher salt, and 1/2 teaspoon black pepper. Set aside until needed.
Place the fish, skin side down, in a roasting pan. Thickly coat the top side of the fish with the olive spread. Layer the tomatoes and cilantro flat on top of the tapenade like scales, then top with the onions and garlic. Place the fish in the oven and bake until the juices run clear and the onions are beginning to crisp and slightly blacken along the edges, about 20 minutes.
Remove the fish from the oven and let rest for 15 minutes. Place the fish on a platter and serve, garnished with cilantro sprigs.
In the bowl of a food processor, combine all the ingredients and puree. Transfer to a bowl. Cover and set aside until needed, or refrigerate for up to 3 days in an airtight container.
Yield: 1 1/2 cups