How to Make It
In a large saucepan, melt 3 tablespoons of the butter over moderately high heat. Add the radish greens and scallions and cook, stirring occasionally, until wilted, about 4 minutes. Add the water, bring to a boil and simmer until the vegetables are tender, about 10 minutes. Let cool slightly. Puree the soup in batches in a blender, about 2 minutes per batch.
Gently reheat the soup in a clean saucepan. Swirl in the remaining 3 tablespoons of butter and season with salt and white pepper. Serve in shallow soup plates, garnished with the Croutons.