In a large pot, sauté fennel and onion in oil or butter until softened.
Add water, chicken base, coriander, anise, nutmeg, salt and pepper. Bring to a boil, then reduce heat and simmer for 30 minutes, or until fennel is very tender.
Carefully blend the soup until smooth using an immersion blender or a regular blender (in batches).
Return the soup to the pot and stir in the heavy cream and butter. Heat through, but do not boil.