Prep: Preheat oven to 250°F. Set a metal cooling rack inside a large rimmed baking sheet (to keep pancake batches warm); set aside.
Grate onion and potatoes: Using the medium holes of a box grater or food processor fitted with grating disc (preferred), grate the potatoes followed by the onion (no need to separate). Transfer potatoes and onion to a cheesecloth (preferred) or clean kitchen cloth; set aside.
Remove water: Firmly twist the cheesecloth/towel over the sink to wring out as much liquid as possible. Repeat several times until barely any liquid wrings out. Transfer onion/potatoes to a dry kitchen cloth and pat dry; set aside.
Make batter: In a large bowl, add the eggs and whisk. Whisk in the flour, cornstarch, baking powder and spices. Add the potato and onion mixture and stir to coat, then use clean hands to completely combine, but don’t overwork. Note: Mixture should be thick and a little wet but not runny. If the mixture is runny, add more flour, 1 teaspoon at a time, until the mixture thickens. If the batter becomes wet/separates while making the pancakes, gently stir to recombine.
Drop and Flatten: Heat 1/8-1/4-inch oil in the bottom of a cast iron skillet over medium-high heat. Once hot, drop three 1/4-cup mounds of batter into the oil, then flatten with back of a spatula to make 1/2-inch-thick pancakes, about 3-inches wide.
Cook: Pan fry the potato pancakes until golden brown and cooked through, about 3 minutes per side, adjusting the heat as needed. Transfer briefly to paper towels to drain, then transfer to the prepared wire rack and keep warm in the oven while cooking the remaining potato pancakes. Repeat until all the batter is gone.
Serve: Serve with freshly cracked salt and pepper to taste (I like more salt), and serve with sour cream and/or applesauce.