"Celery root salad (Selleriesalat) is a staple in Germany in winter. You can buy it ready-made in a jar but my mom always made it for us from scratch. If you like, you can add garlic or fresh herbs, but I prefer it plain."
Combine celeriac pieces, water, and 1 tablespoon salt in a saucepan, bring to a boil and cook until celeriac is soft, 15 to 20 minutes. Drain, reserving cooking liquid, and set aside to cool, about 1 hour. Cube celeriac and place in a bowl with onion.
Stir together cooled cooking liquid, vinegar, and sugar. Pour over celeriac and onion. Add oil and mix well. Season with salt.