In a large saucepan cook the onion in the oil over medium low heat stirring until softened. Add zucchini and salt and pepper to taste. Cook covered, stirring occasionally for 4 minutes. Add broth and 1 1/4 c. Water and simmer covered for 10 minutes or until zucchini is tender. Remove pan from heat and add corriander and let cool. Puree in processor in batches, transferring to bowl. Add lemon and salt and papper to taste. Serve room temperature or chilled. 4 cups or 4 servings.