Wash and peel the potatoes. Place the potatoes in bowl with cold water to prevent from browning.
Cut potatoes into quarters lengthwise, then cut into 1-inch pieces.
Cut half of the Spanish onion into a medium-small dice.
Sprinkle the potatoes and onion with a pinch of salt.
Heat olive oil in a large saut pan over high heat. When the oil is very hot (almost smoking), turn off the heat, add the potatoes and onion to the pan, and saut for 1-2 minutes.
Put the pan with the potatoes and onion in a 500 degree oven. Remove pan from the oven and stir mixture every 5 minutes to prevent potatoes from burning. Repeat this for 30 minutes.
Once the potatoes and onions have cooked in the oven for 30 minutes, drain out the oil in a strainer or colander.
Crack the eggs and beat them with a whisk until a pale, yellow color.
Add the strained potato mixture to the eggs and stir to incorporate everything to a consistent texture. Taste the mixture for seasoning. Add salt if needed. (Be careful when eating raw eggs!)
Add a generous amount of olive oil to another saut pan and heat until smoking.
When the oil is hot, add the eggs, potatoes and onion.
. Flip and stir the mixture in the pan to prevent the eggs from burning. Use a rubber spatula to separate the mixture from the sides of the pan and to smooth the top.
. Put the pan back into a 500 degree oven for about 5 minutes, or until the surface is firm to the touch.
. Meanwhile, drizzle a plate with olive oil. After the 5 minutes have passed, remove the pan from the oven and place the oiled side of the plate on top. Holding the plate and pan together, flip over so that the tortilla comes out onto the plate. Slide the tortilla back into the pan, so that the other side is now facing up.
. Allow the tortilla to cook in the oven for another 5 minutes.
. When the tortilla is firm to the touch, remove the pan from the oven, and flip back onto the plate as before. Transfer the Tortilla Espanola to a serving platter, cut into slices and serve hot or cold.