How to Make It
In a large bowl, toss the celery root with the sugar and 1 teaspoon of salt; let stand for 15 minutes. Drain the celery root well in a colander and squeeze out some of the excess liquid. Wipe out the bowl.
In the same bowl, whisk the olive oil with the vinegar, Dijon, capers and horseradish. Add the celery root, apples, celery ribs and leaves, fennel, parsley and oregano; toss well. Season with salt and pepper and toss again. Serve right away.