Spanish Omelette

Heat 1/2 cup olive oil in a 10-inch nonstick skillet over medium heat. Add the thinly sliced potatoes and finely chopped onion, and cook for 15-20 minutes, stirring occasionally, until the potatoes are tender but not browned.
Drain the potato and onion mixture in a colander, reserving 2 tablespoons of the oil in the skillet. Set the drained mixture aside to cool slightly.
In a large bowl, whisk the eggs with 1 teaspoon salt and 1/2 teaspoon black pepper until well combined. Add the slightly cooled potato and onion mixture to the eggs and gently stir to combine.
Heat the reserved 2 tablespoons of oil in the skillet over medium heat. Pour the egg and potato mixture into the skillet, spreading it evenly with a spatula. Cook for 5-7 minutes, or until the edges are set but the center is still slightly runny.
Carefully flip the omelette by placing a large plate over the skillet, inverting the skillet to transfer the omelette onto the plate, then sliding it back into the skillet. Cook for an additional 3-5 minutes, or until the omelette is fully set and golden brown on both sides.
Transfer the omelette to a serving plate and let it cool for 5 minutes before slicing into wedges. Serve warm or at room temperature.