1. Mix the garlic, onion, lemon rind, lemon juice, thyme, coriander, cumin, vinegar and oil together in a large, shallow, non-metalic dish, whisking well until thoroughly combined.
2. Tread the pieces of lamb onto 16 wooden skewers and add to the dish, turning well to coat. Cover with clingfilm and leave to marinate in the refrigerator for 2-8 hours, turning occasionally.
3. Preheat the grill to medium. Drain the skewers, reserving the marinade. Cook under the hot grill, turning frequently and brushing with the marinade, for 10 minutes, or until tender and cooked to your liking. Serve immediately, garnished with orange slices.