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Put the spring onions, ginger, sesame oil, 2 tbsp of soy sauce, the mirin, sugar, half the sesame seeds and the peppercorns in a mini chopper and blitz to a paste. Stab holes all over the leg of lamb with a small sharp knife. Put the lamb in a dish and tip the paste over, massage it into the holes and where the meat naturally splits by the bone. If any of the paste falls off, pack back onto the lamb with your hands, then cover and put in the fridge to marinate for at least a couple of hours or overnight.
Take the lamb out of the fridge 1 hr before you want to cook it. Heat oven to 200C/180C fan/gas 6. Place in a large roasting tin and sit the lamb, crust-side up, on the rack. Pour any marinade into the roasting tin with 100ml water, then place in the oven. After about 30 mins, drizzle the crust with a little oil to help it crisp slightly, and if at any point it’s becoming too brown, cover loosely with foil. The juices in the bottom of the pan will caramelise and become sticky, but top up with water if necessary so that they don’t burn. Cook the lamb for 1 hr 30 mins if you like it pink by the bone but cooked. If you prefer it rare, cook for 10 mins less; if you prefer it more cooked, give it 20 mins extra.
About 10 mins before the lamb is done to your liking, scatter with the remaining sesame seeds and continue to roast. When the lamb is cooked, carefully lift it onto a board and add the remaining soy to the roasting tin. Taste and add a splash more mirin, soy and water if it needs it. Bring to the boil and scrape any crusty bits off the bottom of the tin, then pass through a sieve into a gravy jug. Serve the lamb with traditional roast accompaniments or with lettuce leaves, shredded vegetables and chilli sauce for a crowd to wrap up and enjoy.