1 medium red onion, cut lengthwise into 1/2-inch-thick wedges
3/4 pound cherry tomatoes
6 tablespoons olive oil, divided
4 (6-ounces) skinless mahimahi fillets (1 inch thick)
1 tablespoon red-wine vinegar
2 teaspoons black or green tapenade (olive paste)
Accompaniment: plain or parmesan couscous or white rice
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.