2 small baking potatoes (about 3/4 pound total), peeled
1 small onion
1 medium celery root (about 1 1/4 pounds), peeled
2 large eggs
1/4 cup all-purpose flour
1 1/2 teaspoons salt
Vegetable oil, for frying
2 Granny Smith apples, 1 cored and thinly sliced crosswise
1/2 cup sour cream
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.
In each of 2 large nonstick skillets, heat 1/4 inch of oil until shimmering. For each pancake, add a scant 1/4 cup of the batter to a skillet and flatten slightly; you should be able to fit 8 pancakes in each skillet. Cook the pancakes over moderately high heat until golden and crisp, about 3 minutes per side. Transfer to the prepared wire racks to drain.
Meanwhile, grate the whole apple and squeeze until dry; transfer it to a small bowl. Stir in the sour cream. Spoon the apple sour cream onto the pancakes, garnish with the apple slices and serve.