Heat the oil in a deep frying pan or flameproof casserole. Add the chicken and brown on all sides for 6–8 minutes over a medium heat. Remove and set aside on a plate.
Cook the onions in the pan for 5 minutes or until golden brown, stirring regularly. Add the garlic and curry powder or paste and cook for 1 minute, stirring well. Add the stock, tomato purée, sugar and seasoning. Mix well and add the chicken. Bring to the boil, then reduce the heat, cover and simmer for 45 minutes or until tender, stirring occasionally.
Remove the chicken to a serving dish using a draining 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 yogurt. Check the seasoning, then pour the sauce over the chicken. Scatter over the chopped coriander.