Preheat the slow cooker if necessary; see the manufacturer's instructions. Heat the butter and oil in a frying pan, add the onion and fry, stirring, for 5 minutes or until the onion is just beginning to soften.
Stir in the fennel seeds and cook for 1 minute to release the flavour. Mix in the carrots, fry for 2 more minutes, then stir in the orange rind and juice. Tip into the slow cooker pot. Bring the stock to the boil in the frying pan, add salt and pepper, then pour into the slow cooker pot. Cover with the lid and cook on low for 6–8 hours or until the carrots are tender.
Transfer to a liquidizer and purée, in batches if necessary, until smooth, then return to the slow cooker pot. Alternatively, purée the soup still in the slow cooker pot with a stick blender. Reheat if necessary in the covered slow cooker pot for 15 minutes.
Ladle the soup into bowls and serve with drizzled cream and croutons.