How to Make It
Peel potatoes and onion; cut potatoes in half crosswise. Grate potatoes and onion on the large holes of a box grater to make long strands.
Transfer grated potato-onion mixture to a colander placed over a large bowl; press mixture firmly to release liquid. Let stand 5 minutes.
Transfer potato-onion mixture to a square of cheesecloth or a clean kitchen towel; twist and squeeze to remove excess liquid, and discard liquid.
Stir together potato-onion mixture, flour, eggs, and pepper in a large bowl. Just before frying, stir in salt. (Salt draws out moisture, so adding it last ensures the mixture doesn’t become watery.)
Heat 1/8 inch schmaltz in a 12-inch cast-iron skillet over medium. Spoon 2 tablespoons batter for each latke into hot schmaltz, spacing 2 inches apart. Press lightly with a spatula to flatten.
Fry until golden and crisp on one side, 3 to 4 minutes. Use two spatulas to carefully turn latkes; fry until crisp on other side, about 3 minutes. Drain; season with salt, and top as desired.