Preheat the oven to 220°C. Mist a baking sheet with the cooking spray.
Peel the potatoes and then grate on the large holes of a box grater. Transfer to a fine-mesh sieve and squeeze out as much liquid as possible into a bowl. Let the liquid settle for about 10 minutes, and then pour off the watery liquid, reserving the potato starch.
Beat the egg white in a large bowl until it holds a soft peak. Gently fold in the potatoes, spring onions, reserved potato starch, 1/2 teaspoon salt and 1/4 teaspoon pepper.
Drop the mixture by tablespoons on the prepared baking sheet, spacing the latkes about 2 inches apart. Gently spread and flatten into 2 1/2-inch rounds. Roast until the bottoms are browned, 8 to 10 minutes. Flip and roast until the potatoes are cooked through and the bottoms are spotted brown, 4 to 6 minutes.
Serve with the apple sauce and sour cream.