Cut leaves from radishes and wash well. (Reserve radishes for another use.) Melt butter in heavy large saucepan over medium heat. Add onion and saute until tender, about 5 minutes. Add radish leaves and saute until wilted, about 2 minutes. Add potatoes and 3 cups water. Cover and simmer until vegetables are tender, about 20 minutes. Puree soup in batches in blender. Return to saucepan. Mix in milk. Stir over medium heat until hot. Season soup with salt and pepper. Serves 6. Bon Appetit May 1994