Mix potatoes, egg, flour and onion, if adding, until smooth. Season. The mix should be easy to pour but not runny. If so, add a little flour.
Heat oil in pan on medium heat. Add a tablespoon of batter and smooth to desired thickness. Continue around the pan. Depending on your pan you'll probably have six including one in the middle.
Cook 2-3 minutes each side, or until golden brown. Easiest way to flip is with a spoon and knife. It's best to keep it slow and take your time as you don't want it to brown too quickly, and you need the potato and onion to cook through.
Drain on paper towel while you cook the next batch.
Serve hot or cold. Can be reheated nicely either in the pan or in the microwave.