Place the parsley, capers, anchovy fillets, garlic, mustard and vinegar in the bowl of a small food processor. Process until the parsley is well chopped, then drizzle in the oil through the feed tube and season. Place in a small bowl, cover and keep in the fridge until ready to use.
Preheat the oven to 200°C. Grease a roasting dish.
Open the lamb out, flesh-side-up and season with salt and freshly ground black pepper. Place on 3-4 rosemary sprigs, then close up and tie with kitchen string to hold its shape. Using a sharp knife make small incisions in the fat of the lamb, about 5-7cm apart. Place a garlic slice and 2-3 rosemary leaves in each slit. Rub the lamb with oil and season.
Line the base of a roasting dish with the potato and leeks. Place the lamb on top and pour in the white wine. Cover the roasting dish with a lid of baking paper and foil.
Place in the oven and roast for 30 minutes. Lower the oven temperature to 180°C, remove the foil and baking paper lid and baste the lamb with its cooking juices. Cover the roasting dish with the lid of baking paper and foil and continue to roast the lamb for a further 1 hour. You may need to turn on the grill and brown the lamb once cooked.
Remove lamb from the oven, cover and leave to rest for 10 minutes in a warm place. Carve the lamb and serve with the leeks and potatoes and the herb and vinegar sauce. A bowl of steamed green vegetables will complete the meal.
Serving size: 4-6 Recipe courtesy of www.recipes.co.nz