The Spanish tortilla is basically the world's most perfect egg dish. Thick, golden, loaded with potatoes and onions — eat it warm or pack it cold for later.
Peel and slice the potatoes into 1/8-inch thick rounds; pat dry to remove excess moisture for crispier results.
Heat the olive oil in a 10-inch nonstick skillet over medium-low. Add potatoes and onions with 1/2 tsp salt. Gently stir, cover partly, and cook 14–16 minutes, stirring occasionally to prevent browning.
While the vegetables cook, whisk eggs thoroughly in a large bowl with remaining salt and pepper until completely homogenous.
Drain potatoes and onions in a colander, reserving 2 tbsp oil. Let cool slightly, then gently fold into eggs to avoid scrambling them.
Heat reserved oil in the skillet over medium. Pour the egg-potato mixture in, spread evenly, and reduce heat to low. Cook, running a spatula around the edges, for 7–8 minutes, or until almost set but still slightly runny in the center.
Flip: Invert onto a large plate, then slide back into the skillet to cook the other side for 4–5 minutes more, until set and light golden. Rest 5 minutes before slicing.