Put the turmeric, yoghurt and a pinch of salt in a bowl and mix well. Add the chicken pieces and leave to marinate for an hour or so.
Meanwhile, make the paste. Place a frying pan over a low heat and add oil, followed by ginger and garlic. Fry for about a minute, then add all the paste ingredients and fry for 5-6 minutes until lightly coloured. Leave to cool, then put the mixture in a blender, add 50-70ml water and blitz to a fine paste. Set aside.
Place a heavy-bottomed non-stick pan heat and add the 3 tbsp of oil. When hot add the birds eye chillies, letting them sizzle for about 10 seconds. Tip in the onion and a pinch of salt and fry for 10-12 minutes or until browned.
Add the pureed tomatoes and stir well, cooking for 3 minutes. Now add the spice paste and fry for a 4-5 minutes, stirring well, until the sauce thickens slightly and the oil leaving the sides of the pan.
Now turn the heat up slightly and add the marinated chicken pieces, stirring well to coat them with the spice paste. Fry for 4 minutes until lightly coloured. Add the water and season to taste. Simmer, covered, for 25 minutes until the chicken is cooked through.
Add the coconut milk, stir well and simmer for a further 5-7 minutes or until the sauce is slightly thickened.