Peel and roughly chop the onions then combine all the ingredients except the lamb and potatoes in an electric blender or food processor and process until smooth. Cut the lamb into 2" (5cm) cubes and place these, along with the yogurt-cream mixture into a large, heavy pot (preferably non-stick). Bring to a boil over moderate heat then cover with a tight-fitting lid, reduce the heat and simmer very gently for 2 hours.
Check the stew and stir frequently, add in a little milk or water as needed if the sauce becomes too thick. Meanwhile, peel and quarter the potatoes. After 2 hours, add the potatoes to the Rogani Gosht and continue cooking gently for a further 30 to 45 minutes until they are tender. Serve hot.