Peel and cut Darling Clementines crosswise into ½ inch thick slices, and arrange them on a large serving platter.
Trim the fennel bulbs, slice thinly crosswise and arrange over the clementines.
Thinly slice the sweet onions over the fennel.
Mix the oil and balsamic vinegar, drizzle over the salad mixture and season with pepper.
Sprinkle walnut pieces over salad and serve.