Make Dressing: In a bowl, whisk together all the ingredients. Dressing may be made 1 day ahead and chilled, covered.
Trim stalks from fennel and with a mandoline, thinly slice fennel bulb crosswise. With a sharp knife, cut a slice from top and bottom of each orange to expose flesh and arrange cut side down on a cutting board, cutting from top to bottom, remove peel and pith. Cut oranges crosswise into 1/4-inch thick slices.
Arrange fennel and orange decoratively on a platter and scatter with mint. Whisk dressing and drizzle over salad.