Wash, drain well and set aside the chicken pieces.
Place garlic, ginger and chillies with a pinch of salt in a pestle and mortar. Grind to a smooth paste. Add the meat masala, turmeric and water and mix well to form a smooth curry paste.
Heat oil in a wide, heavy pan. Add cumin, black mustard and fenugreek seeds until the seeds sizzle and pop. Add the cinnamon quill, cloves and cardamom pods and heat through for 5 seconds. Next add the onions and curry leaves and fry until the onions caramelize.
Add the curry paste and fry, stirring often until the water has evaporated and the oil separates from the paste.
Add the chicken (and potatoes if using) and give it a good stir. Increase the heat, add salt and cook on high for 5 -6 minutes to seal the meat.
Lower the heat, cook covered until the chicken (and potatoes) are cooked through. Cook uncovered for a further 5 minutes until the juices have evaporated and the curry is nice and dry. Garnish with coriander.
Serve with rice, dahl and tamarind chutney on the side.