In a medium bowl, combine lemon juice, horseradish, mustard, crème fraîche, salt, and 1/4 tsp. pepper.
Thinly slice kohlrabi with a handheld slicer, then stack and cut into matchsticks, mixing it into dressing as cut. Repeat with apple. Stir in half of parsley and transfer to a serving bowl. Sprinkle with remaining parsley and more pepper.
Make ahead: Up to 3 hours, chilled airtight.