Step 1Peel the celery root with a chef's knife. Cut into 1-inch-long matchsticks. Place in a bowl and toss with half the lemon juice and mustard.
Step 2Whisk the remaining lemon juice and mustard with the sour cream and the mayonnaise.
Step 3Add half the dressing to the celery root and toss until coated. Season well with salt and pepper and add more lemon juice or mustard if desired.
Step 4Toss the salad with the parsley and capers and chill at least 1 hour for the flavors to meld. Serve the remaining dressing on the side.