Combine sesame seeds, rind, and salt in a shallow dish. Drizzle oil over fish. Dredge fish in sesame mixture. Place the fish on a broiler pan coated with cooking spray. Broil 12 minutes or until the fish flakes easily when tested with a fork.
While the fish is cooking, bring water to a boil in a medium saucepan; gradually stir in couscous. Remove from heat, cover and let stand for 5 minutes. Fluff with a fork. Stir in onions, juice, and salt. Serve with fish.