Put the bones in a large saucepan or stockpot and pour in 2 1/2 litres of cold water. Bring to the boil, skimming off any scum that rises to the surface, then add the vegetables, bouquet garni, peppercorns and salt.
Partially cover the pan with a lid, reduce the heat and simmer for 2 hours, or until the liquid has reduced by just over half.
Remove the pan from the heat and strain the stock, discarding the bones and vegetables. Leave to cool, then skim off the fat that has risen to the surface.
Store in a plastic container in the fridge for up to 4 days or freeze for up to 6 months.