Make the Nonya curry powder
Roast all the spices, apart from the turmeric in a dry pan until fragrant. Allow to cool before grinding in a pestle and mortar or spice grinder. Mix in the turmeric. Keep what you need for this recipe (5 tbsp) and store the rest in an air-tight jar.
Make the curry
Whizz the onion, garlic and ginger with 1-2 tbsp water in a small food processor to make a paste.
Heat the oil in a shallow, wide pan over a medium heat and add the onion paste. Fry for a few minutes without letting it colour, stirring regularly. Add the curry powder and fry for 3-4 minutes until the oil is released.
Stir in the coconut milk and bring to a simmer. If you are using chicken thighs, add now (chicken breasts need less cooking so add nearer the end).
Throw in the lemongrass, lime zest, tamarind water and potatoes with a good pinch of salt. Bring to the boil and simmer for 20-25 minutes, uncovered, stirring occasionally. If using chicken breast, add 10 minutes before the end of cooking. Add the tomatoes 5 minutes before the end. Check the potatoes are tender before turning off the heat. Adjust seasoning if necessary. Remove lemongrass before serving.
Serve the curry in individual bowls sprinkled with the coriander leaves and red chilli