Spanish Tortilla

Peel and slice the potatoes and onions. The thinner they are the better. It is better to use a mandoline if you have access to one.
Using a medium skillet, heat up the olive oil on medium heat and wait for the oil to shimmer. Add the sliced onion, potatoes, and salt into the skillet. With a wooden spoon, toss the vegetables for about 6-9 minutes. Reduce heat until the oil is bubbling at a slow pace.
Toss potatoes gently every few minutes, until they are tender when pierced with a small knife. Do not let the potatoes break or brown, reduce the heat if necessary.
As the potatoes cook, beat eggs with some salt and pepper in a large bowl.
Drain potatoes in a colander, reserving oil. Wipe out skillet and heat the skillet over a medium flame for a minute. Add two tablespoons of the reserved oil. Gently mix the potatoes in with the beaten eggs and let the potatoes soak for 15 minutes in the eggs. Pour the mixture into the skillet. As soon as the edges firm up, reduce the heat to medium low and cook for another five minutes.
Using a flat wooden spoon, insert around the edges to give it shape and make it easy to slide off, the top should still be raw. Place a big plate on top of the skillet and flip it so the tortilla is now on the plate. Slide the tortilla back onto the skillet and use the spatula to place it back on. Serve warm or at room temperature.