Scrape the potatoes, cut them into thick slices and chop the onion.
Warm the extra-virgin olive oil in a large skillet. Add the potatoes and onions, then cover the pan partially and cook over low heat for about 30 minutes, stirring now and then, until the potatoes are tender. Strain the potatoes and onion through a colander into a large bowl and set the strained oil aside for later.
Beat the eggs separately, then stir them into the potatoes adding the parsley and plenty of salt and pepper.
Heat a little of the strained oil in a smaller pan and tip everything into the pan. Cook on a moderate heat and using a spatula to shape the omelet into a cushion. When is almost set invert it on a plate, slide it back into the pan and cook a few more minutes. Before finishing invert the omelet twice more, cooking it briefly each time pressing the edges to keep the cushion shape.
When is ready slide it on to a plate and cool for 10 mins before serving.
