Peel the cleaned russet potatoes. Grate the potatoes on a box grater over a bowl of cold water. Grate the yellow onion on the box grater into the bowl of water, as well.

Take large handfuls of the potato and onion mixture and squeeze out any excess water. Place it all into a clean kitchen towel and squeeze out and excess liquid you couldn't get out with your hands. Pour out almost all of the water you used to submerge the grated potatoes and onions. You'll notice the white starch that has settled on the bottom of the bowl. Drain all of the liquid but save the starch on the bottom.

Put the dried potatoes and onions back into the bowl with the starch. Add the egg, salt, and pepper. Mix to combine.

Heat your frying oil in a cast iron pan on medium high heat. Once hot, scoop a large spoonful (or use a portion scoop) and carefully lay it into the hot oil. Press it down slightly to create a circle. Fry until you see the edges are gold brown and flip it over to fry the other side.

Drain the latkes on a paper towel and season lightly with salt. Keep warm in a 200F oven. Serve alongside sour cream.