In a small bowl, combine crème fraîche, horseradish, 2 tsp. lemon juice, and 1/4 tsp. kosher salt. Chill.
Cut peel from celery root with a sharp knife. Thinly slice crosswise, then stack slices and cut into thin matchsticks.
In a salad bowl, toss celery root with 1 tbsp. lemon juice. Add arugula, about 1/2 cup horseradish dressing, and remaining 1/2 tsp. kosher salt, and gently toss to coat. Sprinkle with sesame seeds.
Make ahead: Dressing, chilled, up to 1 week.