Preheat the oven to 220°C/425°F/Gas 7. Remove the legs from the bird and cut them in two through the knee-joint. Remove the wings and the suprêmes and cut these into two also. (To make white stock for use on another occasion, put the carcass into a pot with some carrots, lemons, herbs and onions, cover with cold water, bring to the boil and simmer for 2 hours.)
Season the joints lightly with salt and freshly ground black pepper and brown in a little oil in a frying pan. Peel the sweet potatoes and cut into 2cm (¾in) dice, mix with the herbs, a little salt and pepper and half the oil and put in a ceramic roasting dish. Wipe the lemons and cut them in half lengthways, then slice finely. Place the leg joints on top of the potatoes, sprinkle with the lemon pieces and roast for 30 minutes. Add the suprêmes, drizzle the remaining oil on them and cook for a further 30 minutes. The chicken is cooked if the juices run clear when the flesh is pricked.