In a large saucepan cook the onion in the oil over moderately low heat, stirring until it is softened, add the zucchini and salt and pepper to taste, and cook the mixture, covered, stirring occasionally, for 4 minutes. Add the broth and 1 1/4 cups water and simmer the mixture, covered, for 10 minutes, or until the zucchini is tender. Remove the pan from the heat, stir in the coriander, and let the mixture cool. In a blender or food processor purée the mixture in batches, transferring the soup as it is puréed to a bowl. Stir in the lemon juice, season the soup with salt and pepper, and serve it at room temperature or chilled.