Melt butter in a large pot over medium heat. Add leeks and onion, cook until softened, about 5 minutes.

Add garlic and cook for another minute until fragrant.

Add cauliflower florets and broth. Bring to a boil, then reduce heat and simmer for 15-20 minutes, until cauliflower is very tender.

Use an immersion blender to puree the soup until smooth. Alternatively, carefully transfer to a blender in batches.

Stir in milk, cream, and nutmeg. Simmer gently for 5 minutes.

Season with salt and white pepper to taste.

Serve hot, garnished with chopped chives or green onions and a drizzle of olive oil.