Peel the celery root and cut into matchstick shapes about 3 inches long. Put the celery root in a bowl. Salt lightly, then add the lemon juice and toss well. Cover tightly and refrigerate for at least an hour, or overnight.
Drain the celery root and blot with towels. Dress with the crème fraîche, mustard, and horseradish. Add salt and pepper to taste—a pinch of cayenne is good too. Sprinkle with finely slivered chives.