2 bunches radishes, with tops
1 garlic clove, peeled and chopped
1 bunch scallions, all green and white parts chopped
3 cups buttermilk
1 cup sour cream
1 1/2 teaspoons salt
1/2 teaspoon white pepper
1/2 cup olive oil
Trim tops from the radishes and rinse them well, discarding any that are discolored or wilted. In a 1-quart saucepan full of rapidly boiling, salted water, blanch tops just until they are tender, for about 30 seconds. Drain and transfer to a bowl full of ice water to stop cooking. Squeeze dry and chop coarsely. Reserve 2 tablespoons to make infused oil.
Quarter all but 4 radishes. In a food processor, coarsely grind quartered radishes with most of blanched tops, the garlic and scallions.
Transfer mixture to a large bowl or pitcher, add the buttermilk and sour cream and stir well to combine. Season aggressively with the salt and white pepper. Chill for a couple of hours.
While soup is chilling, blend reserved 2 tablespoons blanched radish tops with the olive oil until smooth; strain it through cheesecloth into a bowl. Do not press or squeeze solids, or resulting oil will be cloudy.
To serve, taste and adjust seasoning for salt and pepper. Thinly slice remaining radishes. Divide soup among 6 chilled bowls. Float sliced radishes on top and drizzle with a little green oil.
Makes 6 servings.