Preheat oven to 250°C/525°F/Gas Mark 7. Rinse inside of chicken and dust with 2 teaspoons salt. Stuff cavity with onion, garlic and herbs, then place chicken, breast-side down, in a pre-heated lightly oiled roasting dish.
Brush chicken with a little more oil, sprinkle with salt and pepper, add 250 ml(8 fl oz) of boiling water to the dish and roast for 15 minutes.
Meanwhile, place kumara in a deep saucepan and cover with water. Boil 10 minutes, then drain.
Place remaining oil in a small pan and add spices, seeds and chilli and sesame seeds. Fry over a moderate heat until the seeds begin to pop, then remove from heat.
Combine the spicy oil with the kumara and 1 teaspoon salt and place around chicken in the dish, turning chicken over so that the breast now faces upwards. Brush skin with molasses, turn the oven down to 170°C/340°F/Gas Mark 3-4 and continue roasting until chicken and kumara are cooked – about 45 minutes. If the skin begins to burn, cover the bird with foil.