Separate the radishes from the greens, reserving both. Cook the onion in the butter until translucent, then add the radish greens. Cook over low heat, stirring occasionally, until wilted. Puree in a blender or food processor along with half the yogurt and half the chicken stock. Return to the heat and add the other half of the yogurt and half the chicken stock until heated through and through.
The striking effect of this pink and green swirled soup is typical of the California flair for giving humble and familiar ingredients a new treatment.
