Heat the oil in a deep, heavy-based casserole dish and brown the lamb on both sides for about 5–6 minutes.
Place the remaining ingredients in a food processor and blend until smooth. Season well.
Pour this mixture over the lamb and bring to the boil. Cover tightly and cook in a preheated oven at 150°C (300°F), Gas Mark 2, turning the lamb occasionally, for 2½ hours or until the lamb is meltingly tender and most of the liquid has evaporated.
Remove from the oven and allow to stand for 10–12 minutes before serving, cut into thick slices.