3 medium fennel bulbs, cored and cut into small pieces, feathery tops chopped
1 large red potato, peeled and diced
5 cups chicken stock or canned low-sodium broth
1/2 cup heavy cream
Salt and freshly ground pepper
Melt the butter in a large saucepan. Add the onion and the ground fennel and cook over moderate heat until the onion softens, about 3 minutes. Add the garlic and cook for about 30 seconds, stirring constantly. Add the pieces of fennel bulb, potato and stock and bring to a boil; simmer until the fennel is tender, about 15 minutes.
Working in batches, puree the soup in a blender or food processor. Strain the soup into a clean saucepan, stir in the cream and season with salt and pepper. Serve the soup in bowls, sprinkled with the chopped fennel tops.