Peel and cut the Potato (2) into 1/4 inch slices.
Cut the Onion (2) into 1/4 slices.
Heat 2/3 of the Olive Oil (1/2 cup) in a 8-9 inch skillet (nonstick helps). Add the slices of potatoes in a single layer, covering the bottom of the pan, then alternate layers of potato with layers of onion.
Cover the pan and cook on medium-low heat, lifting and turning the vegetables occasionally until the potatoes are fork tender. This will take approximately 30-45 minutes.
Beat the Egg (5) in a bowl, until slightly foamy.
Once the vegetables are tender, lift them out with a slotted spoon and drain in a colander.
Once the vegetables have drained, season with Salt (1 teaspoon) and Ground Black Pepper (1/4 teaspoon), lift and turn the potatoes and onions to combine.
Add the potatoes and onions to the beaten eggs, pressing them down until they are completely submerged. Let mixture stand for 10-15 minutes.
Wipe out the pan, then heat 2-3 tbs of the olive oil over medium-high heat. Add the egg mixture to the pan and quickly spread it out with a spatula. Cover and reduce the heat to medium-low. Shake the pan occasionally to prevent it from sticking.
Around 8 minutes, gently lift the edges of the tortilla – the potatoes should begin to brown underneath and the edges will shrink slightly. Cook for approximately 2 more minutes or until golden brown underneath. Remove from heat.
Very gently, using the thinest and most flexible spatula you have (I use a fish spatula), work your way around the tortilla carefully working the tool all along the sides and then underneath to loosen the tortilla.
Place a plate (slightly larger than the pan) on top of the skillet. Using oven mitts, quickly turn the tortilla upside down on to the plate.
Over medium-low heat add the remaining 2-3 tbs of oil in the pan, then slide the tortilla back into the pan to cook the other side – about 3-4 minutes.
Transfer the tortilla to a plate or board and serve!