Heat olive oil in a large nonstick skillet over medium-low heat.

Add potatoes and onion and cook gently for 20 minutes, stirring occasionally, until very soft but not browned.

Drain excess oil, reserving 2 tablespoons in the pan.

In a bowl, beat eggs with salt and black pepper.

Add potatoes and onion to the eggs and mix gently.

Return mixture to the skillet and cook over low heat for 6-8 minutes until set on the bottom.

Flip the tortilla carefully and cook for another 4-5 minutes until fully set.

Slide onto a plate and let rest briefly before slicing.