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.