Preheat the oven to 400°F (205°C).
Shred the potatoes and onion using a box grater or the shredding attachment of a food processor.
Transfer the shredded mixture to a colander placed over a bowl. Squeeze the mixture by hand or use a plate with a weight on top of it to drain as much liquid as possible over the course of a few minutes.
Let the liquid in the bowl settle for a few minutes, then carefully pour off and discard the water, reserving the starchy sediment at the bottom.
Place the drained potato and onion mixture in a food processor and pulse a few times to break the shreds into smaller pieces, but do not purée. Transfer back to the bowl with the reserved starch and stir in the additional potato starch, matzo meal, salt, and white pepper until thoroughly combined.
In a heavy, ovenproof 12- to 13-inch skillet (preferably cast iron), heat the oil over medium-high heat for 90 seconds. When the pan and oil are hot, carefully spoon the potato mixture into the hot oil, spreading it evenly to the edges of the pan. Using a spoon, drizzle a small amount of hot oil from the sides of the pan over the top.
Transfer the skillet to the preheated oven and bake for 45 minutes, or until the top is crusty brown and the center is cooked through.
Serve hot or warm, garnished with coarse salt and parsley leaves. Accompany with applesauce and vegan sour cream, or other Passover fixings.