Season the lamb with salt and pepper. Heat the oil in a large ovenproof casserole dish. Add the lamb and cook, sealing on all sides, for 2 mins. Pour in 2 litres of water, then add the herbs and the whole onion. Bring to the boil and simmer for 20 mins, removing the scum from the top of the liquid with a slotted spoon. Add the vegetable cubes to the pan and bring to the boil again. Simmer on a low heat for 40 mins. Remove the lamb, add the sliced leeks and simmer for 15 mins. In the meantime, tear the meat apart into strips with two forks, then return it to the pan. Add the shredded cabbage, cover the pan with a lid and simmer for 5 mins. Remove the herbs, season and serve.