Put the onion with the chilli, garlic, ginger, cashew nuts and curry paste in a blender, add 80 ml (1/3 cup) of water and process to a smooth paste. If you don't have a blender, finely chop everything before adding the water.
Heat the oil in a casserole dish over medium heat. Add the blended mixture, season with salt and cook over low heat for 1 minute, or until the liquid evaporates and the sauce thickens. Add the lamb and brown all over. Add the coconut milk and slowly bring everything to the boil. Cover the casserole tightly, simmer for 1 and a half hours, or until the meat is very tender. Stir the meat occasionally to prevent it from sticking to the bottom of the pan.
Serve with rice. I also like to serve with green beans.