Preheat the slow cooker if necessary; see the manufacturer's instructions. Blend the onions, garlic, ginger and chilli in a food processor or liquidizer or chop finely.
Cut each chicken thigh into 4 pieces. Heat the oil in a large frying pan and add the chicken a few pieces at a time until all the meat has been added. Cook over a high heat until browned. Drain and transfer to a plate.
Add the butter to the frying pan. When it has melted, add the onion paste and cook over a more moderate heat until it is just beginning to colour. Stir in the cumin and fennel seeds, cardamom pods and ground spices. Cook for 1 minute, then mix in the stock, sugar, tomato purée and salt. Bring to the boil, stirring.
Transfer the chicken to the slow cooker pot, pour the onion mixture and sauce over the top and press the pieces of chicken below the surface of the liquid. Cover with the lid and cook on low for 5–7 hours.
Stir in the cream. Garnish with toasted flaked almonds and sprigs of coriander and serve with plain boiled rice.