Season each rack of lamb with salt and pepper. Now brown each rack of lamb in a non-stick pan with groundnut oil for 5-8 minutes, turning frequently. Allow the lamb to cool thoroughly.
Mix the marinade ingredients in a small bowl and rub the mixture on the racks with a rubber spatula. Marinate for 1 hour.
Preheat the oven to 230°C. Place the lamb racks in a roasting pan. Moisten the fresh or dry mint leaves with some water and scatter them over the lamb racks. Reduce the heat to 180°C, cover the lamb racks with foil, and roast for 30 minutes. Remove the foil for the last 5 minutes if you want a more browned and crisp look.
Remove the lamb to a cutting board and allow the racks to rest for 20 minutes.
Skim off the fat from the roasting pan, add the chicken stock, and deglaze over a burner, scraping to remove the flavourful bits. Add sauce mix and stir thoroughly.
Carve the lamb racks, arrange them on a serving platter, and serve with the sauce.