Pour the olive oil into a medium-sized, non-stick skillet (about 24 cm / 9.5 inches in diameter). The oil should be about 1 cm deep. Heat the oil over medium-low heat.
Add the thinly sliced potatoes and onions to the skillet. Cook slowly, stirring occasionally, until they are very soft and tender, but not browned. This should take about 20-25 minutes. The goal is to poach them, not fry them crisply.
Remove the potatoes and onions from the skillet using a slotted spoon and let them drain in a colander. Reserve the excess olive oil for other uses.
In a large bowl, whisk the eggs with a generous pinch of salt. Add the drained potatoes and onions to the bowl and stir to combine everything thoroughly. Let the mixture sit for 5-10 minutes to allow the eggs to absorb the flavors.
Pour a tablespoon of the reserved olive oil back into the skillet. Heat it over medium heat. Pour the egg and potato mixture into the skillet and spread it evenly. Press down slightly with a spatula to ensure it forms a compact shape.
Cook for about 5-7 minutes, or until the edges are set and the bottom is golden brown. Place a large plate (larger than the skillet) over the skillet. Hold the plate and the skillet together firmly and quickly flip the tortilla onto the plate.
Slide the tortilla back into the skillet to cook the other side. Cook for another 5-7 minutes, until the second side is also golden brown and the center is cooked to your desired firmness. The center should still be slightly moist. Remove from the skillet and let it cool slightly before serving. It is delicious served warm or at room temperature.
