Heat the butter and olive oil in a large saucepan over medium heat.
Add the chopped onions and cook until soft and translucent.
Stir in the crushed garlic and cook for 1 minute.
Add the cauliflower florets and pour in the vegetable stock.
Bring to a gentle boil, then reduce the heat and simmer for 20 minutes until the cauliflower is tender.
Blend the soup until completely smooth using a hand blender or countertop blender.
Return the soup to the pan over low heat.
Stir in the double cream, Dijon mustard, nutmeg, salt, and black pepper.
Warm gently for 2–3 minutes without boiling.
Taste and adjust the seasoning if needed.
Serve hot, garnished with chopped chives and parsley.