Place all the masala powder ingredients, except the fenugreek seeds and salt, in a hot dry frying pan, and toast for 1–2 minutes until fragrant. Remove the cinnamon stick and reserve for later.
Add the fenugreek seeds and salt to the mix and grind in a mortar and pestle to a fine powder. Set aside. For the chilli paste, blitz everything together until it forms a smooth paste. Set aside.
To assemble the curry, place a large wide frying pan over a medium heat. Once hot, remove from the heat and add the coconut. Cook – still off the heat – stirring all the time. The coconut will toast quickly, turning light brown.
When the coconut is mostly brown, tip it into the bowl of a food processor or blender and add the water. Blend for about 5 minutes, to make a loose paste.
Next, pour the coconut paste into a sieve set over a large bowl. Squeeze out as much coconut milk as you can, using your hands, and once the solids are as dry as possible, discard them. Set aside the toasted coconut milk.
Heat a little flavourless oil in a large heavy-based saucepan with a lid, add the sliced onions and fry over a medium heat for at least 15 minutes, stirring often, until thoroughly browned.
Next, add the chicken and turmeric and brown the meat (do this in batches if necessary) . Return all the meat to the pan (if you browned it in batches) then add all the dry masala powder and the reserved cinnamon stick.
Cook, stirring, for a couple of minutes, then add all the chilli paste, and cook, stirring, for another couple of minutes. Finally, add the toasted coconut milk. If the meat is not mostly covered with liquid, add a little water to the pan.
Bring the curry to a gentle simmer, cover, and cook for 30–40 minutes, or until the chicken is cooked through. (To test this, remove one of the thighs and cut down to the bone – there should be no trace of pink, the juices should run clear and the meat should come easily away from the bone.)
If necessary, remove the lid during cooking for 10 minutes or so, to reduce the sauce. If it is still too loose, place the plain flour in a small bowl and add a couple of tablespoons of the hot curry liquid.
Beat until smooth, then gradually add to the curry pan. Mix well and cook for 5 minutes or so, to allow the flour to thicken the sauce.
Finally, taste and season with salt.To serve, finish with a squeeze of fresh lime juice and coriander. Eat with rice or flatbreads.