For the marinade, use a heavy mortar and pestle or food processor, and purée the coriander roots, garlic, shallots, chilli and turmeric. Add the pepper, fish sauce and sugar to taste, and mix well. Rub the marinade into and under the skin of the chicken and leave to marinate in the refrigerator for 2–3 hours.
Remove the chicken from the refrigerator and allow it to come to room temperature. Prepare your barbecue for direct grilling over a medium heat. Add the chicken. Seal the chicken, skin side down, until crisp and golden brown. Then lift off the heat and separate the coals to create an indirect method of cooking. If using gas just turn the inside burners off and close the lid. Cook the chicken for 20 minutes or until cooked through.
Just before serving the chicken glaze with lashings of sweet chilli sauce.
While the chicken is cooking prepare your herbs and lettuce and arrange on a platter along with some extra chilli sauce. When the chicken is cooked and rested carve into strips and serve in a bowl alongside the garnishes and allow your guests to build their own lettuce cups filled with treats.