Slice the potatoes into thin, even slices (about 1/8 inch thick). Dice the onion finely.

Heat the olive oil in the nonstick frying pan over medium heat.

Add the potatoes and onions to the pan. Sprinkle with half the salt (1/2 teaspoon). Cook gently, stirring occasionally, until potatoes are tender but not browned, about 12-15 minutes.

While the potatoes and onions are cooking, crack the eggs into a mixing bowl. Beat lightly with the remaining salt (1/2 teaspoon) and black pepper if using.

Once the potatoes and onions are cooked, remove them from the oil using a slotted spoon. Drain them well and add to the beaten eggs. Let the mixture rest for 5 minutes so the flavors meld.

Remove excess oil from the pan, leaving just a thin coating. Pour the egg, potato, and onion mixture back into the pan over medium-low heat.

Cook for 5 minutes, gently shaking the pan to prevent sticking. When the bottom is golden and the edges are set but the center is slightly runny, place a large plate over the pan. Carefully flip the tortilla onto the plate.

Slide the tortilla back into the pan, cooked side up. Cook another 3-4 minutes, until firm but still a bit creamy inside.

Slide the finished tortilla onto a clean plate and let it cool for a few minutes before slicing.