You will definitely want a mandolin for this salad. The fennel is best when it is very thinly sliced. If you don’t have a mandolin, use your largest, sharpest knife. Cut the fennel into paper thin slices, crosswise or lengthwise will both work. Whisk together a lemon vinaigrette with the juice from half a lemon, the olive oil and a pinch of cayenne. Add the fennel and toss thoroughly. Season generously with salt and black pepper. Use a sharp knife to cut the tops and bottoms off of the oranges. Now that the orange has a flat bottom, cut away the peel and white pith away from the flesh of the orange. You can do this in strips, rotating the orange as you cut away each piece. Then cut the oranges into slices. Arrange the orange slices on a platter and garnish with the fennel salad.