Add olive oil to a large skillet over medium heat.

Add potato and onion; they should be mostly covered with olive oil (add a little more oil if needed). Season with 1 ½ teaspoon sea salt. Cook on medium-high heat, maintaining a gentle boil, for 8-12 minutes, turning occasionally, until potatoes are just fork tender. Don't overcook.

Drain potatoes in a colander, reserving oil. Taste and season with more salt, if needed. Allow to cool for a few minutes.

Meanwhile crack eggs into a bowl and season with about ½ tsp salt. Beat eggs together. Add potato and onion and toss to coat.

Add a little bit of oil to the bottom of a 10-inch non-stick skillet over high heat. Once hot, pour potato mixture into it and cook on high heat for 1 minute. Reduce heat to medium-low and cook for about 5 minutes.

Run a rubber spatula along the outer edges of the tortilla to make sure it's not sticking and to help it form its shape.

Once it starts to firm up around the edges and in the center, place a large plate over the pan and flip the omelette onto the plate.

Gently slide the omelette back into the pan and cook on high heat for 1 minute, and low heat for 2-3 more minutes or until done.

It's done when it feels set in the center and a knife or toothpick inserted into the center comes out clean.

Flip the finished tortilla de patata back onto a plate, and serve. Serve warm or at room temperature.