Peel and coarsely grate the Starchy Potatoes and the Onion into a large bowl.
Gather the grated mixture in a clean kitchen towel or cheesecloth and squeeze out as much liquid as you can; let the drained liquid sit briefly so the potato starch settles, then pour off the water and stir a spoonful of the settled starch back into the potatoes for extra crispness.
Transfer the squeezed potato-and-onion to a mixing bowl and stir in the Egg, All-Purpose Flour, Salt and Black Pepper until the batter holds together but is still loose enough to spoon.
Heat a generous layer of Vegetable Oil in a large skillet over medium-high heat until shimmering.
Spoon tablespoons of the potato mixture into the hot oil, flattening each mound gently with the back of the spoon to form 1/4–1/2-inch thick pancakes.
Fry the pancakes until deep golden brown on the bottom, about 3–4 minutes, then flip and cook the other side until evenly crisp and cooked through, another 3–4 minutes.
Transfer finished draniki to a paper-towel-lined plate to drain excess oil and keep warm while you fry the remaining batter.
Serve the pancakes hot with dollops of Sour Cream and an extra grind of black pepper, if desired.
