Heat the olive oil in a large skillet over medium heat. Add the diced potatoes and chopped onion, cooking gently while stirring occasionally until potatoes are tender and slightly golden, about 15-20 minutes.
In a mixing bowl, crack the eggs and season with a pinch of salt and cracked pepper. Whisk the eggs until well combined and frothy.
Drain the cooked potatoes and onions from the oil and add them to the bowl with the whisked eggs. Stir gently to combine, ensuring even coating.
Wipe out the skillet and return to medium heat, adding some reserved oil if needed. Pour the potato, onion, and egg mixture into the skillet, spreading evenly. Cook 5-7 minutes until edges set and bottom is golden brown.
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 5 more minutes until set and lightly browned.
Slide the tortilla onto a serving plate. Garnish with parsley if desired. Let cool slightly before slicing and serve warm or at room temperature.