How to Make It
In a large glass or stainless-steel bowl, whisk together the lemon juice, orange zest, orange juice, salt, and 1/8 teaspoon of the pepper. Add the oil slowly, whisking. Add the fennel and onion and toss. Let stand at least 5 minutes but no more than 1 hour.
To serve, top the salad with the grated Parmesan, or with a few curls of Parmesan shaved from the chunk of cheese using a vegetable peeler. Sprinkle with the remaining 1/8 teaspoon pepper.
Variation In addition to the orange zest and juice, add the segments from one orange to the salad. Using a stainless-steel knife, peel the orange down to the flesh, removing all of the white pith. Cut the sections away from the membranes. Squeeze the juice from the membranes to use in the dressing.
Slicing Fennel by Hand If you are slicing fennel by hand, the easiest way is to cut off the stalks, cut the bulb in half from the top through the root end, lay each half flat-side down, and slice crosswise.