Preheat the slow cooker if necessary; see the manufacturer's instructions. Heat the oil in a frying pan, add the chicken a few pieces at time until it is all in the pan, then fry, stirring, until golden. Remove from the pan with a slotted spoon and put in the slow cooker pot.
Add the onions, green chillies, ginger and curry paste to the pan and fry, stirring, for 2–3 minutes. Pour in the coconut cream or milk, stock and ground almonds. Tear half the coriander into pieces and add to the sauce with a little salt and pepper. Bring to the boil, stirring, then spoon over the chicken.
Cover with the lid and cook on low for 6–8 hours. Stir the korma then ladle into bowls, top with spoonfuls of yogurt, the tomatoes and extra raw onion and the remaining coriander torn into small pieces. Serve with warm chapatis.