Place the lamb on the bone into a saucepan. Add the chopped onion, garlic and bay leaves and cover with the water. Bring to the boil and simmer gently for an hour. Remove the lamb and cut the meat into small pieces - discard the bones, but keep the lamb stock.
In a large saucepan, melt the butter and add the flour.
Cook for a minute, stirring all the time and then add the milk. Simmer the sauce for two minutes, add the cheese, the lamb stock, lamb pieces and vegetables. Simmer gently for half an hour stirring occasionally. Season with salt and lots of black pepper