Juice the lemon using a citrus juicer to get 3 tablespoons lemon juice. Set aside.
In a mason jar, add the freshly squeezed lemon juice and olive oil. Add salt and pepper. Put on the lid and shake to combine until dressing is emulsified. Adjust salt, pepper, and lemon to taste. If your lemon isn’t very ripe, you might want a bit more juice. See note 1.
Using a mandolin slicer, thinly slice the fennel bulb. Cut the apples in half and then half again. Remove the core, then thinly slice all the apples with the mandolin. Thinly slice the celery with the mandolin. Add the fennel, apple, and celery to a large bowl.
Gently toss the salad with the dressing. Right before serving, add the roasted walnuts and freshly grated Parmigiano Reggiano PDO. Enjoy!
Notes
Note 1: Depending on the size of your salad and your preference for dressing, you may want to double the dressing recipe.Storage: This salad doesn’t sit or store well, so avoid making it too far in advance. While the lemon juice will delay the apple from browning too quickly, I wouldn’t recommend making this salad more than 30 minutes in advance.