Dry the potato slices thoroughly using paper towels. In a mixing bowl, combine the potatoes, diced onion, 1 teaspoon of kosher salt, and ½ teaspoon of freshly ground black pepper. Toss to coat evenly.

Heat a large skillet over medium heat and pour in enough olive oil to almost cover the potatoes when added. Adjust the amount depending on the pan size. Add the potato and onion mixture to the hot oil and cook, stirring occasionally, until the potatoes are tender enough to be pierced with a fork, about 10 to 15 minutes. Be careful not to let the mixture brown or stick to the pan.

Using a slotted spoon, transfer the cooked potatoes and onions to a colander set over a bowl to drain excess oil. Reserve 3 ½ tablespoons of the oil for later steps. The remaining oil can be stored in the refrigerator for other recipes.

Preheat your oven to 375°F (190°C). In an oven-safe skillet (8- to 9-inch nonstick skillet is ideal), heat 2 tablespoons of the reserved olive oil over medium-low heat.

In a large bowl, whisk together the eggs and season with the remaining salt and black pepper. Stir in the drained potato and onion mixture, ensuring everything is evenly coated. Pour the mixture into the prepared skillet, spreading it out evenly with a spatula. Cook gently for about 5 minutes, using the spatula to loosen the edges occasionally. The mixture should be mostly set, with just a slight amount of liquid remaining on top.

Transfer the skillet to the preheated oven and bake the tortilla until the top is fully set, about 7 to 10 minutes. Once cooked, slide the tortilla onto a serving plate.

Sprinkle the tortilla with the sliced scallions and parsley leaves. Drizzle with sherry vinegar and a little of the reserved olive oil before serving. This dish can be enjoyed warm or at room temperature.