1. Peel the potatoes, cut them into small (0.5-1 cm) cubes and sprinkle a level teaspoon of salt over them.
2. Finely chop the onions.
3. Heat about 250 ml of olive oil in a pan (24 cm diameter) and add the potato cubes. The potatoes should just be covered by the oil.
4. Fry the potatoes oover medium heat for about 12-15 minutes.
5. Sauté the onions in a second pan with a little olive oil for 10-15 minutes.
6. Put the eggs in a large bowl, whisk and salt them.
7. Use a sieve to drain the olive oil from the potatoes .
8. Add the potatoes and onions to the eggs in the bowl and let them sit for about 3-5 minutes.
9. Heat 2-3 tablespoons of olive oil in the pan, add the tortilla mixture and fry for 2 minutes.
10. Turn the tortilla when the mixture starts to go brown (do not let the eggs set) by placing a plate on top of the pan, flipping it over, and then sliding the turned tortilla from the plate back into the pan.
11. Fry it on the back side for another minute. The tortilla should still be slightly runny on the inside.