Method

1. Gently heat the oil in a large, deep heavy-based frying pan.
2. Add the potatoes and onions and simmer on the very lowest heat, without letting the bottom catch, until just cooked through; this will take about 25-30 minutes, sometimes longer, depending on the thickness of the potatoes. Keep testing them with a knife and gently them from top to bottom every 10 mins to help them cook evenly. Weighing the potatoes down with a plate helps too.
3. Drain the onions and potatoes over a large bowl, keeping the oil, and leave them to cool slightly.
4. Beat the eggs in a large bowl and season well with salt and pepper. In the same frying pan, gently heat a couple of tablespoons of the reserved oil, add the potatoes and onions and pour the eggs over the top.
5. Cook very gently for about 10 mins until the bottom is cooked through but the top is still a bit runny.
6. Place the tortilla under the grill on a low to medium heat or in the oven if your pan is ovenproof, to finish cooking the top.
7. Leave to cool slightly then cut into wedges to serve.