Using a 5-holed peeler or vegetable peeler, shave long strands from daikon until you have about 1/2 cup. Save leftover radish for another use.
In a small bowl, whisk together buttermilk, sour cream, 2 tbsp. roquefort, the salt, and pepper (or put those ingredients in a pint jar and shake vigorously).
Divide greens among 4 salad plates. Drizzle dressing over greens. Garnish with radishes, apple, and remaining cheese.
Make ahead: Dressing, covered and chilled, up to 1 week.