AKA: Don’t call it frittata.
(In Spain this is called a Spanish omelet or potato omelet to distinguish it from a tortilla francesa, or French omelet, which is a plain old run-of-the-mill omelet.)
5 medium-to-large potatoes (preferably red potatoes), peeled and diced in relatively small pieces
olive oil or other oil mixed with olive oil, à vôtre gout (about ¾ cup, according to some recipes)
l onion (not sweet onion), diced
salt, à vôtre goût
as much minced garlic as you and those around you can stand—a clove or two
[the whole event should take 45 minutes to an hour]
While cutting up the potatoes, heat the oil in a 10-inch non-stick skillet over medium heat. Raise heat to medium-high and add the diced potatoes.
While the potatoes are frying, you have time to dice the onion, which you should add to the frying potatoes. Add garlic towards the end of the cooking process.(The potato/onion mixture will take a little while to cook. The potatoes will often, and they can brown a little bit. The mixture will shrink as the cooking progresses.)
Remove cholesterol from eggs (see Escoffier’s 1903 cook book to figure out how to do this). While potatoes and onion are cooking away, break eggs into a large bowl, and beat as though making an omelet (which, if you will recall, is what you are doing). Add salt to the extent that you would salt 5 potatoes and 6 eggs. In 1930 in the United States, that would have been two and ½ cups of salt. Now it’s probably about a tablespoon.
When potatoes are soft, drain with a colander. Reserve oil. Add drained potato/onion mixture to beaten eggs. Mix everything up to coat potato/onion/garlic with egg. Test for undersalting, if you want (and you have no fear of uncooked eggs). (You could test for oversalting, but there’s hardly any point. I suppose you could add sugar.) The mixture will be returned to the skillet, but first…
To prevent sticking, which would truly be a disaster, before returning the mixture to the skillet, add about a tablespoon or two (or three) of the reserved oil and heat at medium to medium-high heat. Transfer the goop to the frying pan and spread evenly over frying pan and cook at medium heat. To make sure the omelet cooks in center, pierce through with fork until touching bottom of pan at several points. Make sure the omelet is not sticking to the pan (you know whether it is sticking by shaking the pan; the tortilla should slide around the pan). After the underside has browned (I tell by smell—about 4 minutes), TURN (FLIP) OVER by putting a plate over the omelet, flipping the skillet over (I do this over the sink), and then transferring the tortilla back to the skillet to cook the uncooked side. Cook for an additional few minutes, and do the same flip-over routine to remove the tortilla from the pan. SERVE WARM OR COLD.
Note that you can really put anything you want in a tortilla, such as chorizo, or green/red/yellow peppers, but the classic version is potato and onion only (some would even say no garlic).