For the Tortilla Española:

1. Peel the potatoes, then use a mandoline or sharp knife to slice them into ⅛- to 3/16-inch thick slices. Cut the onion in half and thinly slice it.
2. Heat olive oil in a large, high-sided 10″ nonstick skillet over medium-low heat. Once shimmering, add the onion and cook for about 5 minutes, until just starting to soften.
3. Add the sliced potatoes and cook over medium-low heat for 18–22 minutes, stirring occasionally, until the potatoes are completely tender when pierced with a knife but have very little color. You’re gently confiting the potatoes in olive oil rather than frying them, so if they begin to brown, lower the heat.
4. Once cooked, using a slotted spoon, transfer the potato and onions to a bowl to cool slightly. Season with salt. You can gently break up any of larger potato slices if you wish, as you’ll want a mix of larger and smaller pieces for the best texture. 
5. Meanwhile, whisk the eggs in a separate bowl until smooth and slightly frothy. Add the potatoes and onions, season with a bit more salt, and gently fold everything together. Taste the mixture and adjust the seasoning if needed.
6. Carefully strain the olive oil into a heatproof container. Return 3 tablespoons of the reserved oil to a 10-inch nonstick skillet and heat over medium-high heat.
7. Add the egg and potato mixture and gently swirl the tortilla until the bottom starts to set. Then, use a silicone spatula to run along the sides of the pan to gently loosen the edges and start forming the tortilla’s puck shape. 
8. After about 4–6 minutes, when the edges are starting to set, it’s time to flip. Place a large dinner plate upside down over the skillet, firmly hold the plate against the pan, and quickly invert the tortilla onto the plate.
9. Carefully slide the tortilla back into the skillet to cook the second side. Use the spatula to gently tuck and shape the edges into a smooth, rounded disc. Cook for another 2–3 minutes, depending on how runny you like the center. The tortilla should feel tender when pressing with a finger to the center.
10. Carefully slide the finished tortilla or invert it onto a clean serving plate. Let it rest for 5–10 minutes before slicing. Serve warm or at room temperature.