Position a rack in the center of the oven and heat the oven to 200°F. Put a paper-towel-lined rimmed baking sheet on the rack.
Fill a large bowl halfway with cold water. Using the large holes of a box grater, grate the potatoes into the water, or grate onto a cutting board and then immediately put them in the water.
Line a colander with cheesecloth and set it in the sink. Using a slotted spoon, transfer the potatoes to the colander; reserve the bowl of water. Gather the cheesecloth into a bundle and squeeze firmly until the potatoes stop giving off liquid.
Transfer the potatoes to a medium bowl and using your hands, mix in the onion, egg, 1 tsp. salt, and 1/4 tsp. pepper. Carefully pour off the reserved potato soaking water to get to the white gluey starch on the bottom of the bowl. Transfer the starch to the potato mixture and mix it in with your hands.
Add enough oil to a heavy-duty 12-inch skillet, preferably cast iron, to measure 1/8 inch deep. Heat the oil over medium-high heat until a potato strand dropped in it sizzles vigorously.
Scoop 1/4 cup of the potato mixture onto a slotted metal spatula and use the bottom of the measuring cup to press it until it’s about 1/4 inch thick, letting any excess liquid fall into a small bowl. Don’t worry if the latke is not perfectly round. Slide it into the oil. Make 1 or 2 more latkes for the first batch and fry, flipping once, until they’re golden-brown, 4 to 6 minutes. Transfer to the baking sheet in the oven to keep warm. Repeat with the remaining potato mixture, stirring it between batches. Serve immediately or cool and freeze for up to 1 week and reheat in a single layer, uncovered, in a 300°F oven.
