Preheat your oven to 120°C/ fan 100°C/ Gas 1/2. Line a baking sheet with foil, and keep in oven until needed.
In a food processor or on a box grater, coarsely shred the potato and onion. For longer, moplike strands, I prefer to lay the potato sideways in the chute of the food processor. Transfer the shredded mixture to a square of cheesecloth or lint-free dishtowel, and gather the ends to wring out as much water as possible. Let it stand for 2 minutes, then squeeze it out again.
In a large bowl, whisk the flour, baking powder, salt, pepper and egg together. Stir in the potato-onion mixture until all the pieces are evenly coated.
In a small, heavy skillet (cast iron, if you have one), heat 2 tablespoons of vegetable oil until it shimmers. Drop one-quarter of the potato mixture into the skillet, and flatten with the back of a spoon to a 5-inch round. Cook the latke over moderate heat until the edges are golden, about 4 to 5 minutes; flip, and cook until golden on the bottom, about 3 to 4 minutes more.
Transfer rosti to the prepared baking sheet in the oven. Repeat process with remaining rosti batter in three batches, creating a total of four large rostis, being sure to add more oil as needed and letting it fully reheat between pancakes. Keep latkes warm in oven until needed. Serve latkes warm in four wedges with eggs or whole with a fried egg atop each.