1. Preheat grill to medium high heat (350° to 400°). Arrange 4 (12-inch-long) sheets heavy-duty aluminum foil on work surface. Divide first 3 ingredients evenly among sheets of foil. Top with halibut fillets; sprinkle fillets evenly with salt and pepper. Divide garlic evenly among fillets; top each with a rosemary sprig, and drizzle with oil. Gather sides of foil over fish and vegetables to cover completely; crimp edges to seal packets.
2. Place fish packets on grill. Cook, turning once, 10 to 12 minutes or until vegetables are sizzling inside packets and fish is opaque throughout. Serve with couscous.