Heat 1/2 cup olive oil in a large nonstick frying pan over medium heat.
Add the sliced potatoes and onions. Cook gently, stirring occasionally, until potatoes are soft but not browned, about 15 minutes.
In a large bowl, beat the eggs and season with salt and pepper.
Using a slotted spoon, transfer potatoes and onions to the egg mixture, mixing gently to combine.
Remove excess oil from the pan, leaving a thin layer for cooking.
Pour the egg, potato, and onion mixture back into the pan, spreading evenly.
Cook on medium-low heat until the edges begin to set, about 5 minutes.
Carefully invert the tortilla onto a large plate, then slide it back into the pan to cook the other side for another 5 minutes.
Once cooked through and golden, slide the tortilla onto a serving plate.
Let it cool slightly before slicing and serving.