Return the onions in the pan to the heat, add the remaining oil, cardamom pods and cinnamon stick, then stir-fry for a couple of mins. Stir in the chicken, ground coriander, 1¼ tsp of the garam masala, mace and black pepper, then stir-fry for another 2 mins. Reserve 3 tbsp of the yogurt, then slowly start to add the rest, 1 tbsp at a time, stirring between each spoonful. When all the yogurt has gone in, stir in the oniony paste and stir-fry for 2-3 mins. Stir in 150ml water, then the milk. Bring to a boil, then simmer, covered, for 20 mins, scattering in the chillies for the final 5 mins, by which time the chicken should be very tender. Remove the cardamom pods and cinnamon. The flavours mellow all the more if refrigerated overnight. When gently reheating, splash in a little water if needed to slacken the korma sauce.