Roughly chop and toast the walnuts in a skillet over medium heat until golden brown and fragrant.
In a large bowl, whisk together all dressing ingredients.
Peel the celery root, core the apple, and coarsely grate both using a food processor or coarse grater. No need to peel the apple, unless you wish so.
Add the grated celery, apple, and toasted walnuts to the bowl and toss well with the dressing.
Cover and refrigerate for 2 hours before serving.
Notes
Make-Ahead: I like to cover and refrigerate it for 2 hours or overnight before serving, but it can be served right away!Leftovers can be stored in the refrigerator for up to 2 days.