Put 5 litres (175 fl oz) water into a very large saucepan or stockpot. Add the chicken and slowly bring to a simmer, skimming regularly to remove any impurities. Simmer, uncovered, over low heat for 2–4 hours. Strain through a fine strainer. Return to low heat and simmer until reduced by three-quarters and rich enough to coat the back of a spoon. Finish with a splash of white vinegar.