In a saucepan, heat the oil. Add the onions and cook over moderate heat, stirring, until softened, about 5 minutes. Add the zucchini and cook, stirring, until sizzling, about 2 minutes. Add the vegetable stock and bring to a boil. Simmer over moderate heat until the zucchini is tender, about 10 minutes.
Working in batches, puree the soup in a blender. Rewarm the soup in the saucepan, season with salt and pepper and whisk in the crème fraîche. Ladle the soup into bowls and garnish with a little sherry vinegar.