Put the lamb into a saucepan into which it just fits and add the onions, leek, thyme, bay leaf and peppercorns. Mix the chicken stock with an equal quantity of water and pour into the pan to cover the meat. (If necessary, add a little more water or dilute stock). Add the salt and bring to the boil. Skim off any scum from the surface, lower the heat and simmer for 1¼ hour