Combine the celery root, fennel, apple and lemon juice in a large bowl and season with salt and pepper. Toss everything together and set aside for at least 30 minutes and up to 2 hours. This not only tenderizes and marinates the vegetables so they become pliable but also stops them from turning brown.
Combine the mayonnaise, sour cream, vinegar, honey and mustard in a small bowl. Season with salt and pepper.
Pour the dressing over the vegetable and toss well. Fold in the dried cranberries. Taste and season with salt and pepper if needed.