Peel and cut the potatoes in cubes; cut the onions in thin slices (or as you wish).
Beat the eggs in a bowl and set aside.
Heat the oil in a deep skillet and, in low heat, saute the potatoes and onions for about 10-15 minutes. Season as you wish; remove excess oil from potatoes n onions then mix these into the eggs.
In a separate skillet, heat 4 tablespoons of olive oil and when hot enough, add egg mixture. Let it set for around 5 minutes (until it starts to harden -- ). You have to flip it (probably the most troublesome part -- I use a large plate to help myself flip the tortilla) and then cook the other side pretty well (for about 5 minutes).
You can serve hot or cold because it's a tapa! :D.