How to Make It
Heat the oil in a large saucepan until shimmering. Add the fennel pieces and onion, cover and cook over moderate heat, stirring occasionally, until crisp-tender, about 7 minutes. Add the chicken stock and potato, cover and bring to a boil. Simmer over moderately low heat until the vegetables are tender, about 10 minutes.
Working in batches, puree the soup in a blender or food processor until smooth. (Alternatively, use an immersion blender to puree the soup.) Return the soup to the saucepan and reheat gently. Season with salt and pepper. Ladle the soup into shallow bowls and mound the crab on top. Garnish with fennel fronds and serve.