Put the oil in a large frying pan or sauté pan and set over a medium—high heat. When hot, put in the cinnamon sticks. Let them sizzle for 5 seconds.
Put in the onion. Stir and fry until it browns at the edges. Add the ginger and garlic. Stirfor 1 minute. Add the lamb. Stir and fry, breaking up the lumps until the meat loses its redness.
Add the yoghurt, tomato passata, cumin, coriander, cayenne pepper and turmeric. Stir for 1 minute. Add the salt, potatoes and 475 ml/16 fl oz water. Stir and bring to the boil. Cover, lower the heat and cook gently for 30 minutes.