1. In a heavy saucepan, sauté onion in olive oil for 5 minutes. Add zucchini, salt and pepper. Cook for 5 minutes or until zucchini softens.
2. Add chicken broth and 1/2 cup water. Simmer for 5 minutes. Add basil and parsley. Puree mixture in a food processor or blender, working in 2 batches.
3. Transfer to a serving bowl. Set bowl in a larger bowl filled with cold water and ice cubes for 20 minutes.
4. Stir in yogurt. Add extra salt and pepper if desired. Ladle into bowls and garnish each with sliced radishes.
Look for zucchini that are about eight inches long—larger squash tend to be tougher. To keep them from going soft, store them in a perforated plastic bag in your veggie crisper and use within three days.