Mix all marinade ingredients together in a large bowl. Add the chicken pieces and mix well to coat them in the marinade. Cover and leave in the fridge for 1-2 hours or overnight if possible.
Melt the butter in a large frying pan. When it starts foaming add the passata. Allow to bubble away, stirring until some of the liquid has evaporated and the sauce is starting to thicken – about 5 minutes. Add the fenugreek and garam masala and mix through. Add the vinegar and salt to taste. (You can turn the heat off and leave to sit until required at this stage).
To cook the chicken, heat 4-5 tbsp groundnut oil in a wok or large frying pan and fry the chicken (in batches if necessary), turning until cooked through.
To serve, add the cooked chicken to the sauce and bring back to a simmer. Add the cream and warm through but do not allow to boil or the cream will split.
Serve with Indian breads and pilau rice.