Cut potatoes into 1cm cubes and put in a large heavy-based frying pan. Cover with olive oil and place over high heat until the oil just starts to warm, then reduce to low-medium and confit for 25-30 minutes.

The oil should not be bubbling rapidly. The idea is to soften the potatoes without turning them into little chips.

When softened, put potato into a sieve and drain the oil, which can be kept and used another time.

Slice onion thinly and put in a small pot with a tablespoon of olive oil and a pinch of salt. Cook on low for about 20 minutes, stirring occasionally, until it has a soft, jam-like consistency.

Meanwhile, in a large bowl, gently whisk the eggs until smooth and mix in 1 teaspoon of salt. Reserve 185ml of the egg mixture in a separate bowl. Add the potato and onion to the egg mix in the large bowl and briefly mix. Season with salt.

Heat 2 tablespoons olive oil in a 26cm non-stick frying pan over medium heat. Pour in the egg and potato mixture. As the mixture starts to thicken shake the pan in a circular motion — the edges should start to round. Using a wooden spatula, start shaping the mixture into a thick disc, like a fat frisbee.

After 4 minutes, cover the frying pan with a plate and quickly flip the plate and pan so the tortilla is on the plate. Gently slide the tortilla back into the pan, uncooked side down, and reduce the heat to low-medium.

Smooth out any imperfections in the cooked surface by pouring over a little of the reserved beaten egg and smoothing it in with a wooden spoon. Cover and cook for 2 minutes.

Flip the tortilla again and repeat the smoothing process with some of the remaining beaten egg. Cover and cook for a further 5 minutes.

Turn over once more and cook for a further 5 minutes.

When done, the tortilla should not be completely firm and should have a little wobble in it when you gently shake the pan.

Slide onto a plate and cover with plastic wrap for 10 minutes. This allows the residual heat to set the remaining uncooked egg without the interior becoming rubbery.

Serve at room temperature.