Also known as a Spanish omelette, the potatoes and onions make this Spanish tortilla crispy and give it tons of flavor and add a great touch to any breakfast.
Line a large plate with paper towels. Heat the oil over medium heat in a 10-inch nonstick skillet. Add half of the potatoes; they should sizzle. Cook, turning carefully, until tender but not dark, about 4 minutes. Transfer to the plate with a slotted spatula. Cook the remaining potatoes the same way. Add the onions to the oil and cook until softened, about 7 minutes. Transfer to the plate with the potatoes. Pour off all but 2 Tbs. of oil from the pan.
Beat the eggs, parsley, salt, and several grinds of pepper in a large bowl. Add the potatoes and onions and mix gently to combine. Discard the paper towels and reserve the plate. Heat the pan with the oil over medium-low heat. Add the egg mixture, pressing gently so it is even. Cook until the edges and bottom are completely set and browned, using a spatula to peek under the tortilla; it should hold together but the top middle will be raw, about 8 minutes.
Slide the tortilla onto the plate. Wearing oven mitts, place the pan upside down over the tortilla and turn over so the plate is on top. Continue to cook the tortilla over medium-low heat until cooked through, about 5 minutes more. Transfer to a cutting board and let cool slightly. Cut into 1½-inch squares, and serve warm or room temperature.