Put the lamb in a glass bowl and add ginger, garlic, turmeric powder, cumin powder and coriander powder. Mix well. Let it marinate in the refrigerator for 3 hrs. Heat the oil heavy pan and dissolve the sugar in it. Add the onions to it and fry for 10 minutes over medium heat. Stir constantly. Then remove the fried onions with a slotted spoon and keep it aside. Put the lamb pieces in the same pan and fry until all the sides are browned. Add potatoes and fry for 5 minutes. Then add the browned onions, red chili powder and salt. Pour a cup of water and bring it to a boil. Cover the pan and reduce the heat and cook for an hour or until the lamb is cooked. Stir occasionally. Add garam masala and stir once. Serve hot.