Place all ingredients in a large stockpot and slowly bring to the boil. Reduce heat to a simmer and cook for 1½ hours, occasionally skimming off any froth that may appear on the surface of the liquid.
Pour the stock through a fine sieve into a large bowl or 2-litre jug to cool. Refrigerate overnight. Using a slotted spoon, skim off the solid fat from the surface of the stock.
Use within two days or freeze in 500-ml batches for later use. Makes 2L.