Pour the oil into a large, wide, lidded pan and set over a medium high heat. When the oil is hot, add the onions and stir and fry until they begin to turn brown at the edges. Add the garlic and stir for a minute.
Add the chicken, turmeric and cayenne pepper. Stir, breaking up all the lumps, until the chicken is no longer pink. Add the yoghurt and stir and cook until the yoghurt is absorbed.
Add the tomatoes, salt and potatoes and stir and cook for a minute. Cover, reduce the heat to low and cook very gently for 15 minutes or until the potatoes are tender. Check seasoning and serve.