Peel the potato(es) and onion(s). Cut both into very thin slices. Place a pan over the heat with oil and fry the potato and onion slices until cooked through. Turn them regularly.

When the potatoes are cooked, pour the oil out of the pan.

Crack the egg(s) into a bowl, season with salt and pepper, and whisk until combined. Mix the potatoes and onion into the eggs. Place a pan over the heat with 2/3 of the olive oil and the egg-potato mixture. Let it cook for about 10 minutes over low heat.

Place a large plate on top of the pan and invert it so that the tortilla lands on the plate. Slide the tortilla back into the pan and cook the other side for another about 5 minutes.

Slide the tortilla onto a nice serving plate and drizzle a little more olive oil over it. Serve warm, lukewarm, or cold, حسب your preference.