Preheat broiler and line a large shallow baking pan with foil.
Toss onion, tomatoes, zucchini and mushrooms with 2 tablespoons oil, 2 cloves of garlic, minced, 1 tablespoon vinegar, 1/4 teaspoon pepper in lined baking pan. Spread evenly in one-half of pan.
Pat fish dry and sprinkle with 1/4 teaspoon each of salt and pepper. Arrange skin side down and spaced apart, next to vegetables in other half of pan. Drizzle fish with 1 tablespoon oil and remaining garlic. Broil 4 to 6 inches from heat until vegetables are charred and slightly wilted and fish is just cooked through, 12 to 15 minutes.
Meanwhile, whisk together 1 tablespoon vinegar, tapenade, 1/4 teaspoon pepper.
Serve fish and vegetables over rice or couscous. Stir pan juices into vinaigrette and drizzle on top.