If you have a mandoline, this would be a great time to use it.Serves four as a main course; twelve as a tapa.
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 min., 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 min. 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 min. 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 min. 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 min.
Transfer the tortilla to a serving platter and let cool at least 10 min. Serve warm, at room temperature, or slightly cool. Cut into wedges or small squares, sticking a toothpick in each square if serving as an appetizer.
