Put the chicken on a board breast-side down with the legs facing you. Using kitchen scissors or poultry shears, cut along either side of the backbone and remove it (freeze it to use for stock). Open the chicken out, turn it over and push down the breastbone to flatten. Cut a few slashes in the legs and thighs.

Whisk the marinade ingredients together. Reserve 2 tbsp and rub the rest all over the chicken, working it under the skin. Cover and chill for at least 1 hr or overnight.

Light the barbecue, stacking the coals on one side. When the flames have died down and the coals are ashen, lay a flameproof foil tray or old roasting tin over the empty side and pour in the ginger beer or 100ml water. Set the grill on top. Place the chicken skin-side up on the grill, cover with the lid and cook for 45 mins. Baste with the reserved marinade, then re-cover and cook for 5 mins more. Move the chicken to the hotter side of the barbecue for 5 mins to char the skin a little, being careful not to burn it. The chicken is ready when the legs pull away easily and a digital probe thermometer inserted into the thickest part of the thigh and breast reads 70C. Lift onto a board, rest for 10 mins then scatter over the spring onions and coriander. Drizzle with sesame oil.