Put 3 tablespoons of the oil into a very large, wide, heavy-bottomed pan and warm over medium heat. Brown the lamb shanks, in batches, in the pan and then remove to a roasting tin or whatever else you've got to hand to sit them in.Peel the onions and garlic and process in a food processor or chop them finely by hand. Add the remaining oil to the pan, and fry the onion-garlic mush until soft, sprinkling salt over to stop it catching. Stir in the turmeric, ground ginger, chilli, cinnamon and nutmeg, and season with some freshly ground pepper.Stir again, adding the honey, soy sauce and Marsala.Put the shanks back in the pan, add cold water almost to cover, bring to the boil then put a lid on the pan, lower the heat and simmer very gently for 1-1½ hours or until the meat is tender.Add the red lentils and cook for about 20 minutes longer without the lid, until the lentils have softened into the sauce, and the juices have reduced and thickened slightly.Check for seasoning.Toast the nuts by heating them for a few minutes in a dry frying pan, and sprinkle onto the lamb as you serve it.