- Heat butter in a large saucepan over medium-high heat.
- Add garlic, fennel and mushrooms.
- Cook for 5 to 7 minutes, or until fennel is translucent.
- Add broth, water, potato and tarragon. Season with salt and pepper.
- Bring to a boil; reduce heat to medium-low and simmer for 15 minutes.
- Pour soup into a blender or use an immersion blender to purée the soup. Divide among 6 bowls. Garnish with a thin stream of cream and thinly sliced mushrooms.
Variation: Replace beef broth by chicken broth; replace water by warm milk, adding to soup just before blending.