Squeeze the excess moisture from the potatoes.
Transfer the potatoes to a large bowl.
Stir in the egg whites.
Then stir in the flour, onions, salt (if desired) and pepper.
Lightly spray an unheated large skillet or griddle with no-stick spray.
Heat over medium heat.
Meanwhile, shape the potato mixture into six 1/4"-thick patties.
Add the patties to the hot skillet or griddle.
Cook about 5 minutes on each side or until lightly browned and crisp.