Peel and thinly slice the Waxy Potatoes (about 2–3 mm thick) and thinly slice the Onion.
Pour a generous amount of Extra-Virgin Olive Oil into a large nonstick or well-seasoned skillet and warm over medium heat.
Add the potatoes and onions to the pan and cook gently, stirring occasionally, until the potatoes are tender and the onions are translucent but not browned, about 15–20 minutes.
While the vegetables cook, beat the Eggs in a large bowl and season with Salt.
When the potatoes and onions are done, use a slotted spoon to transfer them into the beaten eggs and mix gently to combine; let the mixture sit for a few minutes so the flavors meld.
Discard most of the oil from the skillet, leaving about 1–2 tablespoons, then warm it over medium-low heat. Pour the egg, potato, and onion mixture into the pan and spread it evenly.
Cook slowly until the edges begin to set and the bottom is golden, about 5–7 minutes; shake the pan occasionally to prevent sticking.
Use a large plate to invert the tortilla: place the plate over the skillet, flip the pan so the tortilla lands on the plate, then slide it back into the skillet to cook the other side for another 4–5 minutes until fully set and lightly golden.
Slide the tortilla onto a serving plate, let it rest a few minutes, then cut into wedges and serve warm or at room temperature.
