Peel the potato and onion. Slice thinly. It works great if you use a mandolin.
Heat the oil a large non-stick skillet over medium heat.
When hot add in the potato slices and onion slices.
Sprinkle with a good bit of salt and a pepper,
Using a wooden spoon gently turn the mixture in oil and adjust the heat so the oil is just bubbly but not too hot.
Keep turning the potatoes every few minutes until they are fork tender. You want them tender, but not brown.
While the potatoes are cooking you can beat the eggs in a large bowl with some salt and pepper.
Drain the potatoes into a metal colander and reserve the oil.
Wipe out the skillet with a paper towel.
Add in two tablespoons of the reserved oil and heat to a medium flame.
Add the potatoes and onions to the egg mixture and gently combine together.
Add this to the heated oil.
As soon as the edges start to firm, reduce the heat to medium-low. Cook for 5 minutes.
Using a rubber spatula run it along the edges of the tortilla.
You want to make sure it is completely loose and will slide easily from the pan.
Carefully slide it onto a plate. Cover it with another plate and holding them tightly together, invert them.
Add another tablespoon of oil to the skillet and slide the tortilla back into the skillet.
Cook another 5 minutes and then slide onto a clean plate.
Serve warm or at room temperature.