Peel the yellow potato and slice it into very thin, uniform rounds.
Finely dice the yellow onion.
Heat the extra virgin olive oil in a 10-inch non-stick skillet over medium heat.
Add the potato slices and diced onion to the skillet, sautéing for 8-10 minutes until the potatoes are tender and the onions are translucent and fragrant.
In a medium mixing bowl, whisk together the large eggs, egg whites, sea salt, and black pepper until the mixture is light and well combined.
Pour the egg mixture into the skillet over the potatoes and onions, using a spatula to ensure the vegetables are evenly distributed.
Reduce the heat to medium-low and cook for 5-7 minutes, or until the edges are firm and the bottom is a light golden brown.
Place a large flat plate over the skillet and carefully invert the tortilla onto the plate.
Gently slide the tortilla back into the pan to cook the other side for an additional 3-4 minutes until fully set.
Transfer the finished tortilla to a serving board, garnish with chopped fresh parsley, and slice into wedges.