Peel the potatoes and slice them into thin rounds, about 1-2 mm thick. You can use a mandoline slicer to make this task easier.
Peel and thinly slice the onion.
Heat the olive oil in a large non-stick frying pan over medium heat.
Add the sliced potatoes and onions to the hot oil. Season with salt. Cook them gently, stirring occasionally, until the potatoes are tender but not browned. This should take about 10-15 minutes. The potatoes should be soft enough to be easily pierced with a fork.
While the potatoes and onions are cooking, crack the eggs into a large mixing bowl. Beat them well with a fork or a whisk. Season with a pinch of salt and pepper if desired.
Once the potatoes and onions are tender, use a slotted spoon to remove them from the pan, allowing the excess oil to drain back into the pan.
Add the drained potatoes and onions to the beaten eggs in the mixing bowl. Mix them gently to combine and let them sit for a few minutes to absorb the flavors.
Wipe the frying pan clean and return it to medium heat. Add a little more olive oil if necessary to coat the bottom of the pan.
Pour the potato and egg mixture back into the pan. Spread it evenly with a spatula.
Cook the tortilla for about 3-4 minutes on one side until it starts to set and the bottom turns golden brown. To flip the tortilla, place a large plate over the pan, and carefully invert the pan to transfer the tortilla onto the plate.
Slide the tortilla back into the pan (uncooked side down) and continue cooking for another 3-4 minutes or until the other side is also golden brown and the center is set. You can slightly lift the edges to check if the center is cooked.
Once the tortilla is cooked to your liking, remove it from the pan onto a plate. Let it cool for a few minutes before slicing it into wedges.
Serve your Spanish Tortilla warm or at room temperature. Enjoy!This classic Spanish dish makes a delicious appetizer, tapa, or even a light meal.