To make the masala, combine the cashews, chilies, ginger and the cold water and blend at high speed for 1 minutes. Add the cinnamon, cardamom, cloves, garlic, poppy seeds, coriander seeds and cumin. Blend again until the mixture is completely pulverized. Set the masala aside. Place the saffron in a small bowl, pour in boiling water and let soak for at least 10 minutes.
In a heavy skillet heat the ghee over moderate heat until a drop of water flicked into it sputters instantly. Add the onions and, stirring constantly, fry for 7 or 8 minutes, until soft and golden brown. Stir in the salt and the masala, then add the yoghurt. Stirring occasionally, cook over moderate heat until the ghee lightly films the surface.
Add the lamb, turning it about with a spoon to coat the pieces evenly. Squeeze the saffron between your fingers, thin stir it and its soaking liquid into the skillet. Reduce the heat to low, cover tightly, and cook for 20 minutes, turning the lamb cubes over from time to time. Scatter 1/2 of the fresh coriander over the lamb and continue cooking, tightly covered for 10 minutes more, or until the lamb is tender.
To serve, transfer the entire contents of the skillet to a heated platter, and sprinkle the top with lemon juice and the remaining fresh coriander.