Line a large bowl with a clean floursack style dish towel or 4 layers of cheesecloth. Grate potatoes and onion into the center of the towel. Twist ends of the towel together to wring out as much liquid as possible. Set the liquid aside to give the starch time to separate — you'll need it later.
Transfer the grated potato and onion to a bowl. Add in scallions, baking powder, salt, pepper, and egg. Mix well.
When the liquid you drained from the potatoes and onions has separated, pour off the liquid layer on top. Leave the layer of starch in the bottom of the bowl.You may need to let the liquid sit for an additional 15-20 minutes to fully separate.
Add the potato mixture to the bowl with the starch on the bottom and mix until combined. The starch acts as a binder to hold the mixture together.
In a large cast iron or heavy-bottomed skillet over medium-high heat 1½ cups of chicken schmaltz or vegetable oil (about ½" of oil). When a shred of potato dropped into the oil instantly bubbles and sizzles, the oil is hot enough.
Tightly pack potato mixture into ¼ or ⅓ cup measuring cup, pressing out any excess liquid.
Carefully drop it into the oil. Use the back of the measuring cup or a spatula to flatten the mixture into a slightly larger disc.
Fry each pancake until deeply golden brown (4-6 minutes per side). Remove finished pancakes to a paper-towel lined cooling rack for 2 mins, then transfer to a sheet pan in a 200°F oven to stay warm.