1. Boil pomegranate juice in a small heavy saucepan until reduced to about 1/4 cup, about 6 minutes.
2. Transfer to a small bowl or measuring cup and quick-chill in an ice bath.
3. Meanwhile, cut fennel into paper-thin slices with slicer. Cut peel, including all white pith, from oranges. Cut segments free from membranes and coarsely chop.
4. Whisk 1/2 teaspoon salt into pomegranate juice, then whisk in oil.
5. Arrange fennel and oranges on salad plates or put in a large salad bowl. Sprinkle with cheese and drizzle with dressing. Season with pepper.