In a large heavy saucepan, melt the butter in the olive oil. Add the celery, onion, bay leaf and thyme and cook over low heat, stirring occasionally, until the onion is soft, about 20 minutes. Increase the heat to moderate. Stir in the zucchini and stock, season with salt and pepper and simmer the soup until the zucchini is soft, about 10 minutes. Discard the bay leaf.