In a small bowl, whisk together the lemon juice, olive oil, salt, and pepper until the dressing has emulsified.
Thinly slice the fennel bulb, apple, and celery. Using a mandoline can help achieve even slices.
Place the sliced fennel, apple, and celery in a large bowl. Drizzle the dressing over the vegetables and toss gently until well coated. Optionally, sprinkle chopped walnuts overtop. Serve immediately.