Peel the potatoes, dropping each one into a bowl of cold water to prevent discoloring. In a large mixing bowl, beat the egg whites. Add the onions and gradually beat in the flour. Pat the potatoes dry with a paper towel and grate coarsely into a sieve or colander. Press down firmly into the sieve in order to squeeze out as much moisture as possible. Immediately add the potatoes to the egg and onion batter.
Preheat the oven to 250 degrees. Spray nonstick spray into a heavy skillet, heat until very hot, then pour one-fourth cup of the potato mixture into skillet. Flatten with a spatula into a pancake about 5 inches in diameter. Cook for about 2 minutes on each side. When pancake is golden brown and crisp around the edges, transfer to an ovenproof dish and keep warm in the oven. Continue making the pancakes as described, adding a little more nonstick spray to the skillet as needed and allowing it to heat before adding more batter.
Makes 4 servings