One of the most classic and popular of all Spanish dishes, the egg and potato tortilla is, simply, iconic. It was, fittingly, the first dish I learned to make when I moved to Spain in 1996, in a lesson given to me by my future brother-in-law, Robert. Preparing a tortilla with potato alone is fine, but using an equal amount of onions produces a sweeter, moister, and, in my mind, superior result. While the key to a good tortilla is keeping it moist in the center, the real trick, he showed me, comes in flipping it. Or rather, flipping the tortilla without the bottom sticking.
1. Peel the potatoes, halve lengthwise, and thinly slice crosswise. Peel the onions, halve lengthwise, and thinly slice crosswise.
2. In a large sauté pan or deep skillet, heat the olive oil over high heat until shimmering. Carefully add the potatoes and onions and cook over medium-high heat, stirring from time to time, until they soften and just begin to brown, about 15 minutes. Using a slotted spoon, transfer the potatoes and onions to a colander to thoroughly drain. Reserve 2 tablespoons of the oil.
3. In a large bowl, beat the eggs thoroughly with a hand whisk until frothy. Season with salt. Pour the drained potatoes and onions into the egg. Gently push down to cover with egg. Let sit and absorb for 10 minutes.
4. In a 10-inch/25-cm nonstick skillet, heat the reserved 2 tablespoons of oil over high heat. Pour in the egg mixture. Immediately turn the heat to low and swirl the pan in a circular motion for a few seconds to keep the egg from sticking. Cook until the bottom is golden and the tortilla set, about 6 minutes.
5. Wearing an oven mitt, place a flat, tight-fitting plate over the tortilla. Firmly pressing the plate against the pan, carefully and quickly turn the tortilla over onto the plate, and then slide the tortilla off the plate and back into the pan. Swirl the pan in a circular motion to settle the tortilla and keep it from sticking. Tuck any edges down with a spatula. Cook for another 3 to 4 minutes, until firm but still moist in the center.
6. Flip the tortilla onto a clean plate. Dab off any excess oil with a paper towel. Let cool before slicing it into fat wedges to serve.
Jeff Koehler, Spain: Recipes and Traditions from the Verdant Hills of the Basque Country to the Coastal Waters of Andalucía, Chronicle Books (2013).