Preheat the oven to 180C and get a frying pan over a high heat.
Add a dash of oil, add the rosemary and the lamb and stir fry for a minute. Splash over the Worcestershire sauce and kep shaking and stirring the pan to coat the meat in a sticky glaze. When the lamb is sticky, remove from the pan.
In a baking dish put a layer of half of the potatoes and season well. Add the lamb over the top, then another layer of potatoes and season again. Add the stock until just reaching the tops of the potatoes, cover with foil and bake for about 30 minutes. Remove the foil and bake for another 15 minutes, or until the potatoes are tender.