In a medium saucepan, cover the sliced fennel with water. Add the lemon juice and a large pinch of salt and bring to a boil. Cover and simmer over low heat until the fennel is tender when pierced with a knife, about 20 minutes. Drain the fennel.
In a large pot, heat the olive oil. Add the zucchini, onion and garlic, season with salt and cook over moderate heat, stirring occasionally, until just softened, about 10 minutes. Add the chicken stock and cooked fennel and bring to a boil. Cover partially and simmer over moderate heat, stirring a few times, until the vegetables have softened completely, about 8 minutes.
Working in batches, puree the soup in a blender. Return the soup to the pot and season with salt and pepper. Ladle the soup into shallow bowls. Dollop 1 tablespoon of the crème fraîche into each soup and swirl in. Garnish with the fennel fronds and serve.