Spanish Omelet (Tortilla Española)

Chop the potatoes into fine matchsticks. Chop the onion. Fry these together slowly in the oil, without browning, until tender. Add the beaten eggs and fry for a couple of minutes. Now turn the omelet over to cook the other side. A simple way to do this is to place a plate on top and turn the frying pan upside clown, then slip the tortilla back into the pan on the other side.