Peel the potatoes and rinse them under cold water.
Cut them into 1 cm slices.
Place the potatoes into a large bowl, then sprinkle with salt and mix well to combine.
Pour 1 liter of vegetable oil into a large frying pan and heat over medium-low heat.
When the oil is hot, add the potatoes.
Cut the onion into small pieces and add to the frying pan and mix well with a spatula.
Turn the heat down to low and cook for around 25 minutes or until the potatoes and onion are turning golden.
Remove the potatoes and onion from the oil, and place in a strainer over a bowl for around 5 minutes, so the extra oil is drained into the bowl.
While the potatoes and onion are cooking, crack the eggs into a mixing bowl, season with some salt, then whisk the eggs with a fork.
Add the potatoes and onions to the egg mixture and stir well.
Let the egg mixture sit for about 10 minutes.
Place a frying pan over low-medium heat and add oil. When the oil is hot, pour in the egg mixture, and cook for around 7 minutes, with a spatula loosen the sides of the tortilla.
Place a large plate over the pan, flip quickly, and then slide the tortilla back into the pan to cook the other side and cook for another 5 minutes, or until golden and cooked through.
Place another large plate over the tortilla and flip quickly.