Combine the fennel and celery in a bowl, then dress with the lemon juice, olive oil and some salt, tossing very well. Check the seasoning and adjust as necessary. Place on individual plates or a platter and top with the grated Parmesan.
You can substitute fennel or celery with many other options, such as carrots, Brussels sprouts, cabbage, cauliflower or endive. Mix and match to find your favorite combo.