Cook Potatoes: Heat the olive oil in a large non-stick skillet over medium heat. Once it's hot, add the potatoes. Fry them until softened, but not browned. 10-12 minutes. Remove the potatoes from the oil with a slotted spoon and let them drain on a rack or a paper towel lined plate.
Cook Onion: Add the sliced onion to the hot oil and fry for 4-6 minutes, or until softened but not browned. Transfer the onions, using a slotted spoon, to the rack or plate with the potatoes. *Discard all but 2 tbsp of the oil from your pan.
Mix: In a large bowl, whisk the eggs with the salt and pepper. Add the potatoes and onion to the bowl and mix everything carefully, as to not break the potatoes.
Cook: Heat the remaining oil in the same skillet over medium-high heat. Pour the egg and potato mixture into the skillet and spread everything evenly. Cook for 1 minute, then reduce the heat to medium-low. Let the tortilla cook for 8-12 minutes, or until the eggs are set around the edges with a slight jiggle in the center.
Flip: Use a knife or spatula to loosen the tortilla in the pan if needed. Take a large dinner plate and place it, inverted, over the top of the tortilla. With one hand firmly holding the plate down and the other hand holding the skillet handle, carefully flip the tortilla out onto the plate. Then, slide the tortilla back into the skillet, with the cooked side now facing up. Cook for another 4-5 minutes or until the tortilla is completely cooked through.
Cool & Serve: Let the tortilla cool for 5 minutes before slicing and serving. Tortilla can be enjoyed warm or cold.