Heat the oil in a heavy pan and fry the onions until soft. Add the cardamom pods, red chili powder, turmeric powder, cumin powder, coriander powder and nut masala. Stir-fry for 5 minutes. Add yoghurt and tomatoes and bring it to a boil. Then add the lamb pieces and mix well. Fry for 5-7 minutes. Cover the pan and reduce the heat. Cook for about 45 minutes or until the lamb is tender and the gravy is thick. Stir occasionally. Serve hot.