If you like, remove the skin from the chicken joints. Season the pieces with salt and pepper.
Heat the oil in a deep frying pan or flameproof casserole dish over a moderate heat. Add the chicken pieces and brown on all sides for 6–8 minutes. Remove from the pan and set aside on a plate.
Cook the onions in the pan for 15 to 20 minutes until golden brown, stirring regularly. Add the garlic and curry powder or paste and cook, stirring, for 1 minute.
Add the stock, tomato paste, sugar and salt and pepper to taste. Mix well and add the chicken. Bring to the boil, then reduce the heat, cover and simmer for 45 minutes, or until tender, turning the chicken pieces halfway through cooking.
Lift the chicken pieces into a serving dish using a slotted spoon. Heat the sauce over a high heat for 2–3 minutes until reduced and thickened. Take the pan off the heat and stir in the yoghurt. Pour the sauce over the chicken. Scatter with the chopped coriander before serving.