Heat the oil in a large heavy pan.  Add onion, leeks, carrot, parsnips and celery. Cover and cook for 5 minutes.  Add 12 cups (3 litres) of water. Bring to a boil, reduce heat to a simmer.  Add garlic, bouquet garni and peppercorns.  Simmer for 1 hour, skim as needed.

Discard the veggies. Ladle the stock in little batches into a fine sieve over a bowl. Press the solids down with the back of the ladle to get all the liquid out. Set the stock aside to cool. Refrigerate until cold and spoon off any fat that has risen to the top. Use within 48 hours or freeze for up to 6 months.