Rub the lamb leg with rapeseed oil and season with the salt. Sprinkle the whole of the lamb with the milled nori and thyme tips.
Lay the onions, leeks and dried kelp on the base of a large roasting tray. Place the lamb on top of the vegetables and put in a preheated oven of 160˚C degrees.
After an hour, add the vegetable stock in and around the lamb.
Cook the lamb for another 30-45 minutes depending on your likening. After an hour and half the lamb should be rare to medium rare.
Remove the lamb from the oven and allow to rest. Strain the liquid from the tray into a pot. Bring to the boil and then strain through a fine sieve.
Carve the lamb and serve with mash, buttered steamed vegetables and the light broth in warm bowls.