Slice potatoes in thin layers (if you have a food processor, even better)
Finely chop white onion

Put the eggs into a mixing bowl and beat the eggs

Put frying pan in high heat with vegetable oil and fry potato slices.
Due the amount of potatoes, you will have to fry them in at least 2 batches of 7/10 min each aprox. Remove them from the oil and add them into the bowl with eggs
Add the onion with your last batch of potato slices

Whisk the eggs with the fried potato slices, add salt and a pinch of pepper if you like it.

Put a non sticking frying pan in high heat with 2 teaspoons of olive oil and add the mixture. With a plastic turner keep separating the omelette from the frying pan to avoid sticking into the frying pan sides for about 4 to 5 min..

Now, the hard part, put a large dish on top of the pan and quickly turn around the pan. Then, gently, move the tortilla from the large dish back into the pan for another 3 to 4 min.

Let it rest for about 30 min until it's warm/cold (that's when it taste better!) unless you like your food piping hot. In that case, don't wait and endulge yourself. Buen Provecho!!