Heat the oil in the insert pan of a 5 litre (175 fl oz/20 cup) slow cooker or a frying pan over medium heat. Add the onion, garlic, carrot and potato and cook for 10 minutes, or until the vegetables have softened, stirring occasionally. Add the spices and salt and cook for 1 minute.
Return the insert pan, if using, to the slow cooker, or transfer the vegetable mixture to the slow cooker. Add the stock, honey and 125 ml (4 fl oz/½ cup) water. Season well with freshly ground black pepper and stir to combine.
Cover and cook on low for 6 hours, or until the carrot is very tender.
Purée the mixture until smooth, using a hand-held stick blender or food processor. Finely grate the ginger, then squeeze it over a small bowl to extract the juice. Stir the ginger juice through the soup, then season to taste.
Ladle into bowls and serve sprinkled with the mint.