Cut the potatoes lengthwise into pieces and then into thin slices. Place them in cold water and let them soak for 30 minutes, then drain, rinse, and pat very dry.

Heat the sunflower oil and fry the potatoes until they are soft and barely colored. Drain well.

Beat the eggs with water and salt, add the potatoes, and mix everything well.

Heat the olive oil in a pan and pour in the egg mixture. Cook over medium-high heat until the bottom has set, cover the pan, and carefully flip the tortilla.

Slide the tortilla back into the pan and let the other side cook briefly over medium-high heat until it is nicely set.

Serve the tortilla warm, or let it cool to eat later cold.