Tart Granny Smith apple perfectly complements the mellow flavor of celery root (also called celeriac) in this creamy condiment.
Using the large holes on a box grater or the grating disk of a food processor, shred celery root and apples and transfer to a large bowl. In a small bowl, whisk together mayonnaise, mustard, milk, and lemon juice. Pour dressing over celery root mixture. Add walnuts and parsley and toss to combine. Serve at room temperature.
Refrigerate in an airtight container, up to 3 days.