Add the onion to the saucepan and cook over moderate heat until softened, about 4 minutes. Add the fennel and thyme and cook, stirring, until softened, about 5 minutes. Add the stock and simmer over moderately low heat until the fennel is very tender, about 45 minutes. Add the zucchini, cover and simmer over moderate heat until barely tender, about 2 minutes. Discard the thyme sprigs.