Slice the potatoes into thin half-moons, about 3mm thick. Slice the onion thinly if using. Do not cut the potatoes into thick chunks. That gives you a clumsy potato omelette, not tortilla española. The slices should be thin enough to soften fully in olive oil and layer naturally inside the egg, but not so thin that they dissolve into mash. Slicing to this specific thinness matters because it's what allows the potato to become genuinely tender in a reasonable amount of time while still holding together as distinct, visible slices once the tortilla is cut — thicker pieces would need considerably longer to soften through, while paper-thin ones would break down and lose their layered structure entirely.

Add the olive oil to a wide pan and heat over medium-low heat. Add the potatoes and onion, then season with about 6g fine sea salt. Cook gently for 20–25 minutes, turning occasionally, until the potatoes are fully tender and the onion is soft and sweet. The potatoes should not become crispy like fried potatoes. Drain the potatoes and onion through a sieve set over a bowl. Reserve the oil. The potatoes should bend and break easily when pressed with a spoon. They should be tender, glossy, and lightly golden in places, not browned and crunchy. This slow olive-oil cooking is the core technique. Rushing it with high heat gives you browned potatoes before the inside is tender, and the tortilla will eat dry. This gentle, oil-submerged cook behaves closer to poaching than frying — the potatoes soften evenly throughout without the surface ever drying out or crisping, which is exactly the texture that lets them fold seamlessly into the beaten egg afterward.

Crack the eggs into a large bowl. Add the remaining 2–4g fine sea salt and beat just until the yolks and whites are fully combined. Do not whip the eggs aggressively. You want a cohesive custard, not a foamy omelette. Beating only until combined, rather than whisking air into the mixture, is what keeps the finished tortilla dense and custardy rather than light and airy — incorporating excess air here would work against the specific soft, set texture this dish is known for.

Add the warm drained potatoes and onion to the beaten eggs. Fold gently until everything is coated. Let the mixture rest for 10–15 minutes. After resting, the eggs should look slightly thicker and more integrated around the potatoes. This step helps the egg soak into the potato layers and gives the tortilla a better interior. Adding the potatoes while they're still warm, rather than fully cooled, helps them integrate more readily with the egg from the very start, and the subsequent rest gives that combination genuine time to settle before any heat is applied — skipping this pause produces a less cohesive interior once the tortilla is sliced.

Heat a 24cm nonstick skillet over medium-low heat. Add about 15ml of the reserved olive oil. Pour in the egg-potato mixture and spread it evenly. Cook for 5–7 minutes, gently tucking the edges inward with a spatula as they set. Shake the pan occasionally to make sure the tortilla is not sticking. The edges should be set and rounded, the bottom golden, and the top still visibly soft. If the bottom browns too quickly before the center starts setting, the heat is too high. Tucking the edges inward as they set is what builds the tortilla's characteristic rounded, domed profile rather than letting it spread flat and thin against the pan's edges.

Place a large flat plate over the skillet. Hold it firmly and flip the tortilla onto the plate in one confident movement. Slide the tortilla back into the pan, uncooked side down. Tuck the edges again to keep the round shape. No hesitation here. A scared flip is how you make a mess. A single, decisive motion keeps the tortilla's still-soft centre contained within its own structure — a slow or hesitant flip gives gravity more time to work against that soft centre, increasing the risk of it spilling or tearing at the edges.

Cook for another 3–5 minutes for a soft, custardy center, or 6–8 minutes for a more fully set tortilla. Transfer to a plate and let rest for 10 minutes before slicing. For the classic soft center, the tortilla should feel set around the edges but still have a slight gentle give in the center. It should not be liquid, but it should not be dry and rubbery either. This second cook is genuinely shorter than the first, since the tortilla is already largely set — its only remaining job is finishing the second side's exterior and settling the centre to the desired level of doneness.

Cut into 8 wedges. Serve warm, at room temperature, or slightly cooled. The cross-section should show clear layers of potato bound in just-set egg, with a golden exterior and a soft interior. The final 10-minute rest before slicing lets the custardy centre finish settling just enough to hold together cleanly under a knife, producing distinct wedges rather than a centre that spills or smears.