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1-3/4 cups vegetable oil for frying (I use plain olive oil, but never a great extra-virgin)
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1-3/4 lb. (about 5 medium) low- to medium-starch potatoes, such as Yukon Gold, peeled
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2-1/4 tsp. coarse salt
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12 to 14 oz. onions (2 to 3 medium), diced
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5 medium cloves garlic, very coarsely chopped (optional)
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6 large eggs
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1/8 tsp. freshly ground black pepper (optional)
In a 10-1/2-inch nonstick skillet that's at least 1-1/2 inches
deep, heat the oil on medium high. While the oil is heating, slice the
potatoes thinly, about 1/8 inch. Transfer to a bowl and sprinkle on 2
tsp. of the salt, tossing to distribute it well.
When the oil is very hot (a potato slice will sizzle vigorously
around the edges without browning), gently slip the potatoes into the
oil with a skimmer or slotted spoon. Fry the potatoes, turning
occasionally (trying not to break them) and adjusting the heat so they
sizzle but don't crisp or brown. Set a sieve over a bowl or else line a
plate with paper towels. When the potatoes are tender, after 10 to 12
minutes, transfer them with the skimmer to the sieve or lined plate.
Add the onions and garlic (if using) to the pan. Fry, stirring
occasionally, until the onions are very soft and translucent but not
browned (you might need to lower the heat), 7 to 9 minutes. Remove the
pan from the heat and, using the skimmer, transfer the onions and garlic
to the sieve or plate with the potatoes. Drain the oil from the
skillet, reserving at least 1 Tbs. (strain the rest and reserve to use
again, if you like) and wipe out the pan with a paper towel so it's
clean. Scrape out any stuck-on bits, if necessary.
In a large bowl, beat the eggs, 1/4 tsp. salt, and the pepper
(if using) with a fork until blended. Add the drained potatoes, onions,
and garlic and mix gently to combine with the egg, trying not to break
the potatoes (some will anyway).
Heat the skillet on medium high. Add the 1 Tbs. reserved oil.
Let the pan and oil get very hot (important so the eggs don't stick),
and then pour in the potato and egg mixture, spreading it evenly. Cook
for 1 minute and then lower the heat to medium low, cooking until the
eggs are completely set at the edges, halfway set in the center, and the
tortilla easily slips around in the pan when you give it a shake, 8 to
10 minutes. You may need to nudge the tortilla loose with a knife or
spatula.
Set a flat, rimless plate that's at least as wide as the
skillet upside down over the pan. Lift the skillet off the burner and,
with one hand against the plate and the other holding the skillet's
handle, invert the skillet so the tortilla lands on the plate (it should
fall right out). (See How to do the flip for photos
of this process.) Set the pan back on the heat and slide the tortilla
into it, using the skimmer to push any stray potatoes back in under the
eggs as the tortilla slides off the plate. Once the tortilla is back in
the pan, tuck the edges in and under itself (to neaten the sides). Cook
until a skewer inserted into the center comes out clean, hot, and with
no uncooked egg on it, another 5 to 6 minutes.