Prepare the Mediterranean Salsa: Combine all of the ingredients in a large bowl and toss to mix well. Let sit at room temperature for 30 minutes to allow the flavors to blend before serving or cover and refrigerate for up to 2 days.
Preheat oven to 400 degrees F. Rinse the fish well under cold running water (to make sure the cavity is clean of any entrails) and pat it dry. With a sharp knife, make 4 deep diagonal incisions down to the bone on each side of the fish. Sprinkle the cavity with salt and pepper. Lay the onion rings, lime rounds, and rosemary sprigs in the cavity. Close the fish. Insert the garlic slivers into the incisions.
Tie the fish together in 3 or 4 places with butcher's twine to close the cavity. Place the fish on a bed of rosemary branches (or a rack) set in a large roasting pan. Rub the fish with the olive oil; sprinkle with salt and pepper. Roast the fish until completely cooked through, about 40 minutes.
Transfer the fish to a large platter and remove the twine, top with the Mediterranean Salsa. You can also serve this fish on a bed of spinach or arugula leaves with a side of warm orzo tossed with a little chopped fresh parsley and olive oil.
Recipe courtesy of White Dog Cafi, Philadelphia