First, dice up your onions and thinly slice your potatoes. (Peeling the potatoes is optional, but recommended.) I like to do this with a mandolin but you can also slice the potatoes by hand. You want them to be about an eighth to a quarter of an inch thick.
Next, heat up your oil over medium heat and preheat your oven to 350°F. I know it seems like a LOT of oil, but you are going to be shallow frying a LOT of potatoes.
Once the oil is hot, begin frying your potatoes and onions. (To test if your oil is hot enough, either wet your fingers and flick it into the oil - if hot enough it will bubble up a little and spit back at you - or drop in a potato slice. If it starts bubbling, the oil is hot enough.) This will most likely have to be done in a few batches.
While the potatoes are frying, season each batch with a pinch of salt and pepper.
After frying each batch, drain the potatoes in a colander or strainer over a bowl. This way, you can collect the oil that drains off and reuse it for frying the rest of the potatoes.
When you begin frying your last batch - prepare your eggs. Whisk them together in a bowl with what is left of your salt and pepper.
After you have fried all of your potatoes and onions, drain out the rest of your oil (the easiest way that I have found to do this is to just pour it out with the last batch of potatoes and give the colander a good shake so that all of the oil drains into the bowl.) Then, return the pan to the heat.
Scoop up a tablespoon or so of the leftover frying oil from your potatoes and add it back to the warm pan. Swirl it around and make sure the whole pan gets coated with oil.
Now, add the cooked potatoes back to the hot pan and try to spread them out so that they are lying flat.
Pour your egg mixture over the top of the potatoes and let it cook for a few minutes - you want the edges to start cooking. Slide a rubber spatula, around the edges to make sure that the tortilla is not sticking. Then, turn the heat down and cook the tortilla over low or medium/low heat for 5 - 7 minutes. The time will vary based on how hot your pan is.
Now comes the fun part - the flip and slide! Place a large, flat plate on top of your tortilla. (The top is not going to be cooked all the way - that is okay.)
Flip the pan and plate over and the tortilla should drop out onto the plate. (I like to do this over a sink in case any oil drips.)
Then slide the tortilla back into the pan so that the other side can cook. Cook on this side for 2 - 3 minutes, then pop the whole thing - pan and all - into the oven for 5 minutes to be sure the middle cooks all the way.
Finally, slide the tortilla out of the pan and let it cool a bit before serving.