In a large bowl, combine the buttermilk, sour cream, shallot, vinegar and honey; season with salt and pepper. Whisk in the EVOO.
Trim the fennel tops and chop 3 tbsp. of the leaves; discard the remaining tops. Stir the leaves into the dressing. Cut the bulb in half lengthwise and, using a mandoline, shave into thin slices. Add the fennel, radicchio and nectarines to the dressing; toss to coat.