Peel the potatoes and slice them thinly (about 1/8 inch thick). Rinse the slices under cold water to remove excess starch and pat them dry.

In a large skillet, heat a generous amount of olive oil over medium heat. Add the sliced potatoes, seasoning with salt and pepper. Cook for about 15-20 minutes, stirring occasionally, until tender but not browned. If using onion, add it halfway through the cooking process.

In a large mixing bowl, crack the eggs and beat them until well combined. Add a pinch of salt and pepper.

Once the potatoes are cooked, drain them from the oil and let them cool slightly. Gently fold the warm potatoes (and onions, if using) into the beaten eggs, ensuring they are evenly coated.

In the same skillet, add a bit more olive oil if needed and heat over medium heat. Pour the potato and egg mixture into the skillet and cook for about 5-7 minutes until the edges are set and the bottom is golden brown.

To flip the tortilla, place a large plate over the skillet and carefully invert the tortilla onto the plate. Slide it back into the skillet to cook the other side for another 5-7 minutes until cooked through and golden brown.

Once cooked, slide the tortilla onto a serving plate. Let it cool for a few minutes before slicing. Serve warm or at room temperature, either as a main dish or tapas-style.