1.Wrap lamb in prosciutto, secure with toothpicks.
2.Heat large deep frying pan, add oil and lamb, cook until well browned all over. Add carrot, onion, garlic, celery and stock to pan around the lamb, bring to the boil. Simmer gently, covered, for about 20 minutes or until lamb is done as desired. Remove lamb and vegetables from pan with slotted spoon, stand, covered, for 10 minutes. Boil pan juices, uncovered, until reduced to 1 cup (250ml).
3.Meanwhile, boil or steam potatoes and celeriac together until soft, drain. Mash potato mixture with butter, cream and salt and pepper to taste until smooth.
4.Serve sliced lamb with vegetables, pan juices, mash and steamed beans, if you like. Sprinkle with parsley.