For the potato tortillas, peel the potatoes and cut into very thin slices. Peel the onion as well and cut into fine rings. Heat two tablespoons of oil in a frying pan. Fry the potatoes and onion over low heat for 10 minutes, until the potatoes are soft. Remove from heat. Whisk eggs with a little salt and pepper in a large bowl. Mix in potatoes and onion. Heat remaining oil in skillet. Add potato mixture and fry for 5 to 8 minutes, until egg mixture is set.
Hold a plate upside down over skillet and invert tortilla onto it.