Method 

1. Peel the potatoes and slice about 3/4cm thick.
2. Heat 3 tblsp of the olive oil in a large frying pan, then add the onion and cook very gently for 30 minutes. They should be soft, light golden and very sweet. The longer you cook them, the sweeter they will be. You can even leave them for up to an hour if you have the time and a heat diffuser to place under the pan. 
3. Lift the onions out of the pan with a slotted spoon, leaving the oil for cooking the potatoes. Drain the onions over a bowl, as you want them to be as free of excess as possible. 
4. Add the rest of the oil to the pan (it needs to be about 2cm deep). Put in the potatoes, season and cook very slowly in the oil for about 20 minutes. Take care to move them around to not let them burn. 
5. When the potatoes are lightly golden and tender remove the pan from the heat. Lift the potatoes out, again with a slotted spoon or sieve and place over a bowl to drain the oil. 
6. Measure around 4 tablespoons of olive oil from the pan and pour it into a non-stick pan around 3-4cm deep. 
7. Once the potatoes are drained, transfer them to a clean bowl. 
8. Beat the eggs, salt, pepper and add them to the potato bowl along with the onions. 
9. Heat the oil in the pan and pour in the eggs and potato mixture. Cook gently over a medium heat. Keep moving the mixture from the centre to the outside to help cook it evenly for around 1 minute.
10. Leave it to cook slowly for 10 minutes until the base is sealed and light golden. 
11. Hold the pan over the sink (in case of leakage!) and place a large plate or a tortilla turner over the top. Flip it over so that the cooked side is upwards on the plate. 
12. Make sure you still have a good film of oil in the pan (add a little more if necessary) and slide the tortilla, uncooked side downwards into the pan and cook gently for another 5 minutes.
13. When it is ready, the tortilla should be firm but creamy on the inside. To check, press the surface near the centre to make sure that no un-cooked egg seeps out. 
14. Serve on a big flat serving plate with a generous dollop of alioli, a glass of white wine or some delicious olives and a green salad!