In a medium bowl, whisk together the eggs, liquid egg whites, sea salt, and black pepper until well combined.
Heat the extra virgin olive oil in a 10-inch non-stick skillet over medium heat.
Add the sliced yellow onion to the skillet and cook, stirring occasionally, until soft and deeply caramelized, about 8-10 minutes.
Stir in the diced cooked potato and sauté for 2 minutes to allow the flavors to meld.
Pour the egg mixture over the onions and potatoes, shaking the pan slightly to ensure the eggs settle into all the crevices.
Reduce the heat to low and cook undisturbed until the edges are set and the bottom is golden brown, approximately 6-8 minutes.
Place a large flat plate over the skillet and carefully flip the tortilla onto the plate, then slide it back into the skillet to cook the other side for 2-3 minutes.
Alternatively, place the skillet under a broiler for 2 minutes until the top is set and lightly browned.
Slide the tortilla onto a cutting board, garnish with fresh parsley, and let it rest for 5 minutes before slicing into wedges.