Wash, clean, peel, and crack the potatoes roughly so they have irregular borders. It will take you about five minutes. Spanish Omelette
Dice the onion and add it to the potatoes (you can choose not to add the onion). Put all the cracked potatoes and onion in a bowl and add the salt, mix it with your hands and let it rest for ten minutes. It is essential to add the salt at this point, so the potatoes are well seasoned and lose the excess water.
Drain the liquid with a colander or putting your hands on top and slightly flipping the bowl. The added salt helped the potatoes and onion “sweat” their excess water; now, this will prevent it from splashing out as you add it to the hot oil.
Put the olive oil on the heat and ensure it is hot but not extremely hot; otherwise, your onion dices will get burned. Test it with a small potato piece.
Add all the potato and onion mix into the oil and continuously stir to avoid it sticking to the bottom and spread the heat evenly.
When you can see that the potato is soft and cooked, turn the fire up, and keep stirring, so the potato borders get a crunchy edge. This will take about ten minutes.
Add the whole rosemary spring to the oil.
Meanwhile, crack four eggs in a big heat-resistant bowl (don’t stir them).
When the potatoes are a little crunchy or golden, pick them out of the oil, draining them a bit with a strainer, and adding them on top of the eggs.
Stir the mix; you will note that the eggs start to cook a bit. You need a creamy dripping texture so add another one or two eggs if you don’t have a slightly thick and fluid mix.
Take a skillet and add 4-5 tablespoons of the oil you have already used. Turn the heat up. It must be very hot.
Carefully add the egg and potato mix (leaving the rosemary sprig behind), and let it form a crust on the bottom. With a spatula, separate the edges.
When it looks half-way cooked, put a plate on top of it and carefully turn it upside down (be careful, excess hot oil may drip).
Immediately slide the omelette again into the skillet, and let it cook for another 2-3 minutes on low heat.
Slide the omelette out of the skillet into the final plate and serve.