Preheat the oven to 150 degrees C. Cut the lamb into 3cm chunks. Heat the oil in a large flameproof casserole dish, with a capacity of at least 2 litres, over a medium heat. Fry the lamb in batches until it is browned on all sides. Remove with a slotted spoon and set aside.
Cut the potatoes, turnips and leeks into slices about 4mm thick. Using about half the vegetables, place a generous layer of potato slices over the base of the casserole dish, then top with a layer of turnip slices, followed by the leeks. Lightly season between the layers with a little salt and freshly ground black pepper and scatter with the thyme and half the parsley.
Place the lamb on top of the leeks, tucking in the bay leaf. Top with the remaining leeks, followed by the rest of the turnip slices. Finish with the remaining potato slices, arranging them so that they overlap slightly.
Pour over the stock — don't worry if it doesn't completely cover the vegetables as they will cook in the steam. Cover with a tight-fitting lid and bake for 2 hours or until the meat and vegetables are tender and cooked. Just before serving, sprinkle the top with the remaining chopped parsley.