Heat olive oil in a frying pan over Moderate heat. Add onion and Potato and sprinkle with salt. Cook Until soft, but not brown, stirring Add about 1/3 of the beaten egg.
Using a spatula, lift up omelette at the edges and center to allow egg to run under potato and onion. Repeat this procedure until all the egg has been added.
When egg is firm but still slightly moist (not runny) and golden on the bottom, run the spatula under omelette to loosen it from the pan.
Then place a plate over the top and flip omelette onto the plate. (You may want to have someone help you with this.)
Add another tbsp. olive oil to the pan and slide omelette back in, brown side up. Continue cooking omelette over moderate heat until golden on the other side.