Line 2 wire racks with paper towels. Using the large holes of a box grater or the shredding disk of a food processor, grate the potatoes and onion. Working over a small bowl, squeeze the liquid out of the potato and onion and transfer the grated vegetables to a medium bowl. Let the liquid stand for 1 minute, then pour it off, leaving the potato starch at the bottom. Scrape the starch into the potato shreds. Grate the celery root and add it to the potatoes along with the eggs, flour and salt.