Peel and shred the potatoes (I used a food processor to shred) and place into a big bowl of cold water. Let sit for 2 minutes, then rinse well and drain. Squeeze dry with hands.
In a large nonstick skillet, heat ¼ cup of olive oil over medium heat. Add the leeks and season with ¼ tsp salt and ⅛ tsp black pepper. Cook for 10 minutes until the leeks have softened. Remove the leeks to a bowl, and set aside.
Add another ¼ cup olive oil and the grated potatoes to the pan, season with ½ tsp salt and ¼ tsp pepper, and cook for 10 minutes until they are tender. You can toss them during cooking every few minutes in order to cook evenly, but do not mash the potatoes down (it will make them gummy). After 10 minutes, they will be tender but shouldn’t really be browned. Add to the leek bowl and toss to combine.
Whisk the eggs with ¼ tsp each salt and pepper until the yolks and whites are blended. Add the eggs to the leeks and potatoes, and also add the cheese if you’re using it (see notes below). Stir everything to combine.
Heat an 8-inch nonstick skillet** over medium heat, then add 1 tbsp olive oil and swirl it around. Pour in the leek potato mixture, and let it cook for 5 minutes until the sides have set and you can see a browned bottom when you peek underneath with a spatula. Using oven mitts, place a plate on top of the skillet and flip everything over. Then slide the tortilla off the plate and back into the skillet.
Cook for another 3-5 minutes until golden brown and cooked through. If you want to check the inside with a thermometer, egg sets at about 150F. Remove from the pan. Cut into wedges and serve warm or at room temperature. Enjoy!
Feel free to add up to 1 cup of grated Manchego cheese to the batter if you'd like, if you tolerate dairy. Stir it in with the eggs, potatoes, and leeks before cooking.*Do not use more than 2 pounds of potatoes, or you'll have too much filling for the 8" skillet. If you're under a little bit, it's fine, but make sure to weigh them at the store if you don't have a scale since size varies so much.**It's really important to use exactly an 8"skillet that's nonstick. See post for more details.