Heat the butter in a saucepan, add the onion and fry gently for 5 minutes until just beginning to turn golden around the edges. Stir in the potato and cook briefly, then mix in the cauliflower florets, stock, cider and mustard. Season with salt and pepper and bring to the boil. Cover and simmer for 15 minutes until the vegetables are tender.
Mash the soup roughly to thicken it slightly, then stir in the cheese and heat, stirring until melted. Taste and adjust the seasoning if needed, then ladle into bowls. Garnish with chopped chives and serve with croûtons (see