Add butter on a medium to large pot, over medium heat.
Once butter has melted, add onion and celery. Cook until the onion has slightly softened.
Add cauliflower, broth and milk. Mix.
Add thyme and bay leaf. Cover and bring to a boil.
Simmer until cauliflower is tender (about 15 to 20 minutes).
Take pot off the heat and set aside to cool a bit.
Once soup has cooled down, use a food processor (or blender) to puree the soup.
Once puree has reached a smooth texture, pour puree back to the pot. Cover and bring to a boil.
Add crème fraiche. Season with salt, pepper and nutmeg. Cover and simmer for a few minutes. Do not boil, just enough to heat up the soup again after adding crème fraiche.
Serve and garnish with chopped chives, if desired.