Heat the olive oil in a 10-inch non-stick skillet over medium heat, then carefully add the sliced potatoes and onions.

Cook the potatoes and onions gently for about 15-20 minutes, stirring occasionally to prevent sticking, until the potatoes are tender but not browned.

While the vegetables cook, sauté the diced chorizo in a separate small pan over medium heat until crispy and the fat renders, about 5 minutes.

Drain the oil from the potato mixture into a heatproof container (reserve the oil for future cooking) and transfer the vegetables to a large mixing bowl.

In a separate bowl, beat the eggs with salt and pepper until well combined.

Pour the beaten eggs over the warm potato and onion mixture, then fold in the cooked chorizo.

Let the mixture rest for 10 minutes; this allows the warm potatoes to absorb some of the egg and creates a creamy texture.

Wipe out the non-stick skillet, add 1 tablespoon of the reserved oil, and return it to medium heat.

Pour the egg mixture into the skillet and cook for about 5-6 minutes, gently running a spatula around the edges to ensure it doesn't stick.

When the bottom is golden brown and the edges are set, place a large flat plate over the skillet and carefully flip the tortilla onto the plate.

Slide the tortilla back into the pan to cook the other side for another 3-4 minutes, tucking in the edges to give it a rounded shape.

Once the tortilla feels firm to the touch with a slight spring in the center, slide it onto a serving board.

Allow the tortilla to cool for at least 15 minutes to set before slicing into wedges and sprinkling with fresh parsley.