Whisk mayonnaise, creme fraiche, lemon juice, tarragon, sugar, salt, black pepper, and cayenne together in a bowl.
Place apples, celery root, grapes, and walnut pieces in a large bowl. Toss with about 1/2 cup of dressing. Add more dressing until salad components are well coated but not swimming in dressing. You might not need all the dressing.
Serve immediately or cover and refrigerate for up to 24 hours.