Heat the oil in a 8-9 inch skillet. Add the slices of potato one at a time to avoid sticking together.
Alternate layers of potatoes with layers of onions.
Cook slowly over medium heat, turning the potatoes until tender, not brown.
Drain potatoes in a colander, save oil.
Make sure skillet is very clean for later use. In a bowl, beat eggs with a fork until slightly foamy.
Salt to taste. Add the potatoes, press down to cover potatoes completely with egg. Return to skillet, heat 2 tablespoons of saved oil until smoking point. Add the mixture, spread it out. Rapidly, lower the temperature to medium-high.
Shake the pan often to avoid sticking.
When the bottom is brown, cover the skillet with a plate, turn skillet up-side down, remove tortilla, slide off plate with uncooked side down into skillet and cook the other side, Turn several times until cooked