Scrub or peel the potatoes and cut them into even slices.
Peel the onions and cut them into slices.
Heat up the olive oil and add the potatoes. Fry until slightly brown and almost done.
Add the onions to the potatoes and fry until the onion is soft and translucent. You don't want the onions burned,
In the meantime, crack the eggs into a large bowl. Season with salt, and whisk well.
Once the potatoes and onions are cooked, add the mix to the eggs.
Make sure the potatoes and onions are well coated with the eggs.
Use the same frying pan and gently pour the egg mixture into the pan.
Fry on low to medium heat until nicely browned. Use a fork to lift up the omelette in different places so the runny egg touches the pan and can cook as well.
Once is nicely browned, it/s time to flip the omelette/tortilla.
Place a large plate on top of the tortilla and flip it carefully by holding the pan and plate at the same time.
Make sure to wear heat-resistant gloves.
Put the tortilla back into the frying pan with the uncooked side down.
Fry for a few more minutes on low heat. Check if it's cooked by lifting the underside up.
Serve the tortilla cut like a cake (that is how I like it) with a salad on the side.