Place the lamb in a flameproof casserole dish or tagine. Add half the onion, the ginger, cayenne pepper, saffron, ground coriander, cinnamon stick, chopped coriander and some salt and pepper. Cover with cold water and bring to the boil. Reduce the heat and simmer, partly covered, for 1 hour.
Meanwhile, melt the butter in a heavy-based frying pan a