Rinse the fennel, celery and ginger. Scrub the carrots and potatoes (no need to peel). Chop the fennel, celery, carrots and potatoes into medium sized pieces. Peel and grate the ginger.
Saute fennel and celery for 5 minutes on medium-high heat. Add ginger and cook for 2 minutes on medium heat, stirring constantly. Add in carrots, potatoes, stock, pepper and optional salt.
Bring to a boil. Cover and simmer for 15 to 20 minutes, until potatoes and carrots are tender when pierced with a fork.
Puree the soup using an immersion blender or a food processor. Allow the soup to cool. Taste, and add more grated ginger, salt or pepper if desired.
Serve with extra pepper and a dollop of yogurt of choice on top (optional).