How to Make It
In a large skillet, melt the butter over moderately high heat. Stir in the chopped basil, then add the bread cubes and toss to coat with the butter. Season the bread cubes with salt and pepper. Cook, stirring, for about 3 minutes, or until the bread cubes are lightly toasted and crispy. Transfer to a plate.
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.
Working in batches, puree the soup in a blender until smooth. Return the soup to the saucepan and reheat gently. Season with salt and pepper. Ladle the soup into bowls, top with the croutons and basil leaves and serve.