Set a 10-inch non-stick skillet over medium heat with the oil.

Use a mandolin to thinly slice the potatoes and onions. Once the oil is hot, carefully add in the sliced potatoes and onions. Season liberally with salt and black pepper. Cook, stirring often, until the potatoes are tender and the onions have softened, about 8 to 10 minutes. As the potatoes cook, beat the eggs with a good pinch of salt and pepper in a large bowl.

Set a colander over a large bowl or glass measuring cup and drain the potatoes and onion, reserving the oil. Wipe out skillet with a paper towel, set over medium heat and add 2 tablespoons of the reserved oil.

Gently mix the warm potato mixture into the eggs and add to the hot skillet. Use a spoon to spread out into an even layer, as best as possible. After about a minute, once the edges have firmed up a bit, reduce the heat to medium-low. Cook undisturbed for 5 minutes. You're looking for the eggs to have set for the most part but still be a little bit runny. Use a rubber spatula to loosen the tortilla from the sides of the pan. Place a large plate over the top and then carefully, with oven mitts, invert onto the plate. Set the skillet back onto the heat and add in another tablespoon of the reserved oil. Then carefully, slide the tortilla back into the skillet. Cook for 5 minutes and then slide onto a clean plate or serving platter. Serve warm or at room temperature. Enjoy!