Prick the flesh of the racks of lamb all over with a sharp knife. Place them in a shallow, non-metallic dish and set aside.
Place all the remaining ingredients in a food processor and process until you have a fairly smooth paste. Rub this paste all over the racks of lamb, cover and marinate in the refrigerator for 24 hours.
When ready to cook, allow the lamb to come to room temperature. Place the racks on a baking sheet lined with nonstick baking paper, flesh side up, and cook in a preheated oven at 180°C (350°F), Gas Mark 4 for 20–25 minutes or until cooked to your liking.
Remove from the oven and cover with foil. Allow to rest for 5–6 minutes before serving.