Heat a generous amount of olive oil (about 1 cup) in a large frying pan (at least 28cm) over low heat. Add the sliced potatoes and onion with a good pinch of salt. The potatoes should be sliced to about 2–3mm thick—roughly the thickness of a coin—so they cook evenly and soften without breaking apart. Stir gently to coat everything in the oil, then cook gently over low heat for 20–25 minutes, stirring occasionally, until the potatoes are completely tender but not falling apart. They should soften and take on a silky texture without browning.

To test for doneness, try breaking a slice in half with a spoon or spatula—it should give easily with no firmness in the centre.

Using a slotted spoon, transfer the potatoes and onions to a colander set over a bowl to drain. Reserve a few tablespoons of the oil. Let cool slightly.

In a large bowl, whisk the eggs with a generous pinch of salt. Gently stir in the cooled potato and onion mixture, being careful not to break up the slices too much. Let sit for 10–15 minutes to allow the flavours to meld.

Heat 1–2 tablespoons of the reserved oil in a 22cm non-stick or well-seasoned frying pan over medium-low heat (about 4 out of 10 on the dial if using electric or induction). Pour in the egg and potato mixture and gently shake the pan to even out the surface.

Cook uncovered for 6–8 minutes, or until the base is lightly golden and the edges have just set. You want the bottom to cook slowly without burning—adjust the heat down slightly if it starts browning too fast.

While it cooks, preheat your grill (broiler) to medium-high—around 220°C / 425°F if your oven has temperature settings, or one level down from the highest setting.

Once the tortilla has mostly set but the top is still slightly runny, transfer the pan to the grill and cook for 2–4 minutes, or until the top is puffed and lightly golden. Keep a close eye on it—it cooks quickly at this stage.

Let the tortilla rest in the pan for at least 10 minutes before slicing. This allows the eggs to finish setting and makes it easier to cut clean slices. Serve warm, at room temperature, or cold the next day.