A beauriful whole branzino, grilled to perfection, with a no-cook, fresh, and vibrant salsa. Thanks to my father-in-law's golden rule of a hot, well-oiled grill, it never sticks. The salsa pulls double duty as both sauce and a salad.
Make the Tomato-Caper Salsa. In a bowl, combine the diced tomatoes, olives, capers, garlic, red onion, and parsley. Add the olive oil and balsamic vinegar. Toss gently and set aside at room temperature while you cook the fish.
Prep the fish. Pat the branzino completely dry, inside and out. Cut 2 to 3 shallow (1/2-cm.) diagonal slashes into the thickest part of each side. Rub all over with olive oil and season generously with salt, inside the cavity, over the skin, and into the slashes. Stuff the cavity of each with lemon slices, parsley, and a rosemary sprig. Let it sit at room temperature until the grill is ready.
Prep the grill. Preheat a gas grill to medium-high (400–450°F) with the lid down for 10 to 15 minutes. Scrub the grates clean, then oil them well: dip a folded paper towel in oil, hold it with tongs, and wipe the grates until glossy.
Grill the first side. Lay the fish on the hot grates, close the lid, and cook undisturbed for 5 to 6 minutes, until the skin is seared and releases naturally.
Flip the fish. Gently turn the fish once with a wide spatula. Close the lid and grill another 4 to 6 minutes, until the flesh is opaque, flakes at the bone, and reaches 145°F at the thickest point.
Finish and serve. Transfer the fish to a platter and rest 1 to 2 minutes. Remove the head (if preferred) and spoon the tomato-and-caper salsa generously over the top, letting the juices run into the cavity. Serve immediately.