With a very sharp knife, slice fennel bulbs crosswise into paper-thin slices, then cut slices into quarters. Slice and quarter orange similarly.
Put fennel and orange slices in a salad bowl, then crumble in feta cheese and add olive oil and lemon juice. Toss thoroughly, then add salt to taste (if desired) and grind in plenty of black pepper. Toss again and serve on salad plates garnishe