The first time you eat a Spanish tortilla, a thin cake of potatoes and egg, you're amazed that two average ingredients could make such a sublime dish. The sliced potatoes give the tortilla density, while the eggs keep the layers moist. The combination creates an unsuspected sweetness that makes a second, or even a third, piece hard to resist.
Meanwhile, beat three eggs until frothy. Once the potatoes and onions are cooked, drain them, reserving two tablespoons of oil, and add to the eggs. It's important to let the potatoes sit in the eggs for 15 or 20 minutes, since this allows the eggs to penetrate the potatoes.
Add the reserved oil to a pan over medium-high heat. Spread the egg and potatoes across the pan and cook until the bottom begins to brown. Now we come to tricky part: cover the pan with a plate, flip the pan, and slide the now inverted tortilla back into the pan. Many an excellent tortilla has landed in the sink when some hot eggs or oil dripped onto the cook's hand. Don't feel bad if you lose a tortilla sometimes, it happens to everyone. Experienced tortilla makers like to flip it several times to better mold the shape, but beginners should be satisfied with one turn.
Once both sides are brown, slide the tortilla out of the pan and allow to cool. This dish is best served at room temperature. Like a stew, the flavors improve the second day. Some like to smear mayonnaise across the top, and Las Bravas in Madrid covers their tortilla with a spicy sauce, but it's also excellent with no embellishments.