Combine the potatoes, onion, 4 tablespoon olive oil and salt. Toss to coat each potato slice as evenly as you can.
Heat 2 tablespoons olive oil in a nonstick saucepan until shimmering, on medium to high heat. Add the potato mixture and cook, covered and stirring often, until the potatoes give little to no resistance when pricked with a fork.
In the same bowl that the potatoes were tossed in, whisk the eggs, garlic, anchovies and peas. When the potatoes have finished cooking, scrape the potato mixture into the bowl with the eggs and stir to combine.
Clean the pan the potatoes were cooking in and add another two tablespoons olive oil, heat until shimmering and just starting to smoke, over medium to high heat. Add the potatoe and egg combination mixture and cook, shaking the pan in between folding the mixture around, for about 30 seconds. Make sure the egg and potatoes haven’t started to stick to the bottom of the pan and that the eggs are being turned around enough to facilitate even heat distribution. Reduce heat to low, cover and let cook, shaking every other minute or so, until the top has started to set and the bottom has turned a golden brown, about 5 minutes.
Remove from heat and cover the entire pan with an upside down plate. Carefully invert the pan over the plate so that the tortilla is upside down on the plate. Remove the pan and wipe clean. Add one tablespoon of olive oil to the pan and swirl it around before sliding the tortilla back into the pan. You may have to shove slices of potatoes back under the tortilla, but try to keep the whole thing as uniform as possible. Continue to cook over medium to low heat, shaking every other minute, until the tortilla has browned on the bottom. Slide out of the pan and serve hot or cold (I like mine with garlic aioli on the side)