Put the celery, carrot, leek, onion, herb bundle, peppercorns and the onion skin, if using, in the slow cooker – the onion skin will give the stock a dark golden colour. Pour over
Cover the cooker and cook on LOW for 8 hours. Strain the stock into a large bowl, discarding the solids.
If not using immediately, leave to cool completely, then cover and chill. Keep refrigerated and use within 2 days or freeze for up to 3 months.
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