Peel and thinly slice the Waxy Potatoes into rounds about 3–4 mm thick, and peel and thinly slice the Onion into fine half-moons.
Pour enough Extra-Virgin Olive Oil into a large, heavy skillet to coat the bottom (about 4–6 tablespoons) and warm over medium-low heat; add the potatoes and onions, season with Salt, and cook gently, stirring occasionally, until the potatoes are very tender but not browned, about 20–25 minutes.
Using a slotted spoon, transfer the cooked potatoes and onions to a bowl or colander to drain and let cool slightly, reserving the oil in the pan for cooking the tortilla.
In a large bowl, lightly beat the Eggs with a fork or whisk until combined.
Add the drained warm potatoes and onions to the beaten eggs and gently fold to combine, letting the mixture sit for 3–5 minutes so the flavors meld and some egg soaks into the potatoes.
Wipe the skillet down lightly if there are large bits, then return a tablespoon or two of the reserved oil to the pan and warm over medium heat; pour in the egg-potato mixture and spread it evenly.
Cook undisturbed until the edges are set and the bottom is golden, about 5–7 minutes; loosen the edges with a spatula.
Carefully invert the tortilla onto a large plate, then slide it back into the pan to cook the other side until set and lightly browned, about 3–4 minutes more.
Slide the finished tortilla onto a serving plate, let rest a few minutes, slice into wedges, and serve warm or at room temperature.
