Vegetable Stock

• 4 celery sticks, chopped, with the tops reserved
• 1 carrot, sliced
• 1 leek, sliced and rinsed

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 1.5 l/52 fl oz/<