Preheat the oven to 400°. Working over a large bowl, coarsely shred the celery root and potato on the large holes of a box grater. Using 2 forks, toss the celery root and potato with the onion and salt.
In a large nonstick skillet, heat 1 1/2 tablespoons of the oil. Scoop four 1/2-cup mounds of the vegetable mixture in the skillet and flatten into cakes. Cook the pancakes over moderately high heat until browned and crisp, about 4 minutes per side. Transfer to a large baking sheet and season with pepper; keep warm in the oven. Repeat with the remaining oil and vegetable mixture. Serve the pancakes hot.