Mahimahi with Charred Onion, Tomatoes, and Tapenade Vinaigrette

Preheat broiler and line a large shallow baking pan with foil. Toss onion and tomatoes with 3 tablespoons oil, 1/2 teaspoon salt, and 1/4 teaspoon pepper in lined baking pan, then spread evenly in one half of pan. Pat fish dry and sprinkle with 1/4 teaspoon each of salt and pepper, then arrange, skinned side down and spaced apart, next to vegetables in other half of pan. Drizzle fish with 1 tablespoon oil and broil 4 to 6 inches from heat until vegetables are charred and slightly wilted and fish is just cooked thourough, 12 to 15 minutes. Meanwhile, whisk together vinegar, tapenade, 1/4 teaspoon pepper, and remaining 2 tablespoons oil. Serve fish and vegetables over couscous. Stir pan juices into vinaigrette and drizzle on top.