The Spanish tortilla is so much more than the sum of its humble parts. Potatoes are the star, but tortillas welcome variation and flavors with the same technique. The tortilla makes an excellent breakfast, lunch, dinner, snack, or crowd-pleasing tapa.
In a 10-1/2-inch nonstick skillet that is at least 1-1/2 inches deep, heat the oil on medium high. While the oil is heating, slice the potatoes thinly.
Fry the potatoes. When the potatoes are tender, after 10 to 12 minutes, transfer them with the skimmer to the sieve or lined plate.
In a large bowl, beat the eggs, and 1/4 tsp. salt, (if using) with a fork until blended. Add the drained potatoes, and mix gently to combine with the egg.
Heat the skillet on medium high. Add the 1 Tbs. reserved oil. Let the pan and oil get very hot (important so the eggs do not 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.
Set a flat, rimless plate that is 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). 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).
Transfer the tortilla to a serving platter and let cool at least 10 minutes. 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.