Fry onion, garlic, ginger, chilli, cardamom pods and cumin seeds in a little oil until soft and slightly coloured.
Make a paste with the vinegar, turmeric, coriander, cumin, fenugreek and salt. Add to the pan and fry off for a few minutes. Add tomatoes and mix well. Bring to the boil and add chicken and sultanas.
Cook on the hob or in the oven (180C) until chicken is tender (45 mins). Add sweet potato 30 mins before end of cooking if using.