Makes 14 to 16 pancakes; serves 6 to 8
2 pounds Yukon Gold potatoes, peeled and halved
1 yellow onion, cut into 6 to 8 pieces
1 large egg, lightly beaten
6 scallions, including tender green parts, thinly sliced
2 tablespoons all-purpose flour
1 teaspoon salt
1⁄2 cup vegetable oil, plus more if needed
Sour cream for serving
For the batter: In a food processor fitted with the grater attachment, grate the potatoes and onion. Transfer the mixture to a colander and press down firmly to remove all the excess liquid. Then transfer the mixture to a clean kitchen towel and roll it up tightly in the towel. Holding the towel over the sink and grasping one end in each hand, twist the towel to wring out as much liquid as possible.
Unwrap the well-squeezed potato-onion mixture and place it in a large bowl. Add the egg, scallions, flour, and salt and mix well.
To cook the pancakes: Line a rimmed baking sheet or large platter with paper towels. In a large skillet, heat the 1⁄2 cup vegetable oil over medium-high heat until it shimmers. To form a pancake, scoop 1⁄4 cup of the batter into the oil and press down on it with the bottom of the measuring cup to form a round cake about 31⁄2 inches in diameter and 1⁄3 to 1⁄2 inch thick. Repeat to form more pancakes, being careful not to crowd them. Cook, turning once, until golden brown on both sides and cooked through, 3 to 4 minutes on each side. Using a slotted spatula, transfer the pancakes to the towel-lined baking sheet, and tent loosely with aluminum foil to keep warm while you cook the remaining batter the same way.
Transfer the pancakes to a warmed platter and serve hot. Serve with sour cream.