Prep the vegetables. Remove the stalks from the fennel bulb, reserving some of the fronds if you'd like to use them as a garnish. Cut the bulb in half, remove the tough core and slice the fennel as thinly as possible. A mandoline works especially well here.
Build the salad. Add the fennel to a large bowl along with the sliced celery, Castelvetrano olives and red onion.
Make the lemon dressing. In a small bowl, whisk together the extra-virgin olive oil, lemon juice, lemon zest, salt and several grinds of black pepper.
Toss. Pour the dressing over the vegetables and toss well so everything is lightly coated.
Let the flavors mingle. Allow the salad to sit for about 5–10 minutes before serving. This takes a little of the sharpness out of the onion while allowing the fennel and celery to absorb the lemony dressing without losing their crunch.
Finish and serve. Taste and adjust with additional lemon, salt or pepper if needed. Scatter fennel fronds or celery leaves over the top and serve.