The potatoes and onions
I use 1 1/2 pounds of potatoes, about 4 to 5 medium Yukon Golds or Russets. Peel the potatoes and slice them thinly, to about 1/8-inch thick. It helps to have a mandoline for this job. Chop one large onion and toss it together with the potatoes.
One pan is traditionally used for both phases of cooking the tortilla. I didn’t have one that was the right size, shape, and metal to do it all, so I used an enameled Dutch oven for pre-cooking the potatoes and onions and a small cast iron skillet (about 8”) for making the tortilla.
Virgin olive oil—not extra virgin—is the classic choice for tortilla making. Alternatively, you can blend extra virgin olive oil with a neutral high-heat oil (like grapeseed).
The old-school method is to use enough oil to allow the potato slices to separate and swim around a bit—maybe about 1 1/2 cups of oil. With a non-stick pan, Raquel, José, and Tricia get away with less than 1/4 cup of oil.
Cook the potatoes and onions
The oil should be very hot, but not quite as hot as it would be for deep-frying—the potatoes should “simmer” in the oil, not fry.
Salt the potatoes and onions lightly, add them to the oil, and cook for about 10 minutes over medium-high heat, stirring frequently, until the onions are softened and the potatoes are cooked through—a slice of potato should be easily split with a spoon.
Drain the cooked potatoes and onions in a colander, reserving the oil for the next phase of cooking.
While the potatoes are draining and cooling slightly, beat together four eggs and a little salt. Combine the eggs with the potato mixture, making sure all the slices are well coated. Also mix in additions such as bacalao, diced chorizo, ham, artichokes, peppers, mushrooms, or peas. José and Tricia also like to mash some of the potatoes into the eggs.
Many Spanish home cooks will let the potatoes and eggs rest for up to 15 minutes before proceeding. It works out great if you let it sit for as long as it takes you to get your tortilla skillet ready.
Start the tortilla in a very hot skillet
Preheat a small skillet over medium-high heat and add a tablespoon of the reserved oil. Spread the oil evenly round the inside edges and the bottom of the pan. When the pan and oil are very hot, spread the tortilla mixture into the pan, giving it a couple shakes to distribute the potatoes and eggs evenly.
Reduce the heat to medium-low. Run a heat-proof spatula around edges to make sure the tortilla doesn’t stick.
After about 10 minutes, when you give the pan a jiggle and the potatoes don’t slide around and the eggs are just set around the edges, the tortilla is ready to be flipped. Place a plate or pan lid on top of the skillet, pick up the pan and flip it over. It’s that easy.
Cook the second side
Add a little more oil to the skillet and ease the flipped tortilla back in. Shake the pan or use your spatula to adjust the tortilla into position. Raquel likes to flip her tortilla several times—and starts flipping a little earlier than my single flip—to ensure even color and a nice shape.
The total cooking time will vary depending on the thickness of your tortilla. My tortillas generally take about 15 minutes for the eggs to set through the middle.
Serve hot, at room temperature, or cold
That’s part of the beauty of this dish. Eat it whenever and however, you’d like—as a quick snack or part of a sit-down meal. But it does taste best if it’s been allowed to rest for half-hour or more. If you are serving a large group, cut the tortilla into squares. Otherwise, a wedge of tortilla makes a beautiful meal with some Spanish cheese, a simple salad, and maybe even a chorizo.