The recipe (serves 8)
Season the lamb shoulder well and place on a wire rack in a roasting tray. Cover the tray loosely with foil and place in the oven at 120C (Gas Mark 0.5) and slow roast for 8 hours. At Fortnum's, we tend to leave it overnight, for the following morning's shift to take out.Once cooked, pick the meat out while it is still warm, removing all the fat and skin. Shred the meat into decent-sized pieces.Heat a spoonful of lamb fat from the roasting tray in a large pan, add the carrots, celery and onion and cook gently until soft. Stir in the garlic and herbs and cook for a few minutes longer.Next, add the Worcestershire sauce, tomato ketchup and the lamb and mix well. Pour in the stock and simmer for 15-20 minutes, until it has reduced but the mixture is still moist. Check the seasoning.To make the topping, wrap the potatoes in foil and bake at 200C (Gas Mark 6) for about an hour, until tender.Once the potatoes are cool enough to handle, cut them in half, scoop out the flesh and push it through a potato ricer or a sieve into a bowl. Bring the double cream and butter to the boil in a pan and beat them into the potato. Season well, then mix in the egg yolks. Once mixed, put the potato into a piping bag fitted with a star nozzle. Now put the lamb into a large pie dish, pressing it right down. Scatter the frozen peas over, then pipe the duchess potato mix on top, ensuring the filling is completely covered.Finally, place in an oven heated to 180C (Gas Mark 4) for about 20 minutes, until piping hot and golden brown. Serve, and enjoy.