Preheat the slow cooker if necessary; see the manufacturer's instructions. Heat the butter and oil in a large frying pan, add the onion and potato and fry for 5 minutes or until softened but not coloured.
Stir in the cauliflower, stock, mustard, Worcestershire sauce, cheese and a little salt and pepper and bring to the boil. Pour into the slow cooker pot, cover with the lid and cook on low for 4–5 hours or until the vegetables are tender.
Purée the soup while still in the slow cooker pot with a stick blender. Alternatively, transfer to a liquidizer and purée, in batches if necessary, until smooth, then return to the slow cooker pot.
Stir in the milk, replace the lid and cook on high for 15 minutes until reheated. Stir and add nutmeg to taste. Ladle the soup into bowls, swirl cream over the top and sprinkle with a little extra grated nutmeg and some croutons.