Peel and halve celery root, then use a mandolin to thinly slice one half. Cut the other half into matchsticks. Combine celery root with celery stalks, shallot, lemon zest, and horseradish.

Season with salt and pepper, then toss to combine. Allow to rest for about 10 minutes. Meanwhile, whisk oil and lemon juice. Season with salt and pepper.

Drizzle over vegetables, then top with celery leaves and parley, tossing to combine.