Trim any excess fat from the thighs and put them into a heavy-based saucepan with the onions, garlic, ginger, turmeric and yoghurt, stirring until well blended. Place the pan over a medium heat and bring to the boil. Reduce the heat, cover and simmer for 20 minutes, stirring occasionally.
To toast the spices, heat a small, heavy-based frying pan over a medium heat. Add the coriander and cumin seeds, dried chilli and cinnamon and dry-fry them, stirring continuously, for up to 1 minute, or until they release their aromas. Turn off the heat and continue to stir the spices for a few seconds, then transfer them to a plate and allow to cool. When they are cool, grind them finely or with a pestle and mortar.
After the chicken has simmered for 20 minutes, increase the heat slightly, uncover and cook for 5 minutes, stirring frequently, or until the sauce in the pan reduces to a thick paste. Meanwhile, put a kettle on to boil.
Stir in the toasted spices and a pinch of salt, then reduce the heat and simmer, stirring, for 3-4 minutes.
Blend the coconut milk powder with 300 ml of boiling water, then stir it into the chicken mixture and continue simmering, uncovered, for 6-8 minutes, stirring occasionally, until the juices run clear when the chicken is pierced with a knife.
Meanwhile, in a small pan, heat the oil over a low heat and fry the garlic until it has browned slightly. Add the fresh chilli and cook for 30 seconds, then add them to the chicken.
Reserve a little of the chopped coriander and stir the remainder into the chicken mixture with the lemon juice. Remove the pan from the heat, sprinkle the chicken with the reserved chopped coriander, garnish with sprigs of coriander and serve.
Boned chicken can be used instead of chicken thighs with bones, though it has less flavour. If using boned meat, reduce the initial cooking time by 15 minutes.