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1
Pour the olive oil into a 20 cm heavy-based frying pan and warm over low-to-medium heat until it reaches 130–140 °C. Add the sliced potatoes and onion. They should be fully submerged or nearly so. Maintain this temperature — no bubble should be violent, only a slow, gentle sizzle — and confit for 20–25 minutes, turning occasionally, until the potato slices are completely tender, translucent, and sweet, with no resistance when pressed. They must not colour.
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2
Lift the potatoes and onion from the oil with a slotted spoon, draining thoroughly. Reserve the infused oil in the pan. Season the potato-onion mixture with salt. Beat the eggs in a large bowl, season lightly, then stir in one tablespoon of the warm potato-infused oil. Fold the potato and onion through the egg mixture. Rest for 10 minutes at room temperature; by the end, the egg will have begun to thicken around the potato, and the mixture will hold its shape loosely in the bowl.
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3
Pour off all but a thin film of the infused oil from the pan and set over medium heat. When the oil shimmers, pour in the egg-potato mixture and press gently to an even depth of 4–5 cm. Reduce the heat to low and cook for 4–5 minutes. The tortilla lives or dies here: the edges must be set and pulling away from the pan, while the centre, when the pan is shaken, must still tremble and jiggle — a firm, fully set centre is a ruined tortilla. Do not rush this stage.
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4
Place a flat plate larger than the pan face-down over it. In a single, decisive, unhesitating motion, invert the pan so the tortilla drops onto the plate, golden-side up. Slide the tortilla immediately back into the pan, uncooked-side down, and cook for a further 2–3 minutes over low heat. The second side is done when the edges are set and the base, when a palette knife is slipped beneath, releases cleanly and is lightly golden.
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5
Slide the tortilla onto a clean plate and rest for at least 5 minutes before cutting. It may be served warm, at room temperature, or cold; the interior custardy set and the layers fully integrated are the mark of a rested tortilla.