1.Sprinkle lamb with salt and freshly ground black pepper. Wrap lamb in prosciutto; secure with toothpicks.
2.Heat a large deep 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 desired. Sprinkle with parsley.