1. Combine the marinade ingredients in a small pan. Set aside.
2. Clean fish if needed, removing heads, tails and fins, but leave skin on.
3. Cut fish across into 'steaks' 1 1/2 inches thick.
4. Cut onions and zucchini into thick slices, discarding ends.
5. Thread fish, vegetables and lemon slices alternately on 4 kebab skewers.
6. Lay the skewers in a shallow dish and pour the marinade over them. Marinate for 30 minutes, turning several times.
7. Broil kebabs, turning and brushing them with unused marinade, for 6 to 12 minutes or until fish is tender.