5 medium Yukon gold potatoes (1 1/4 pounds)
1 white onion, diced
1 3/4 teaspoons kosher salt, more as needed
3/4 teaspoon black pepper, more as needed
About 1 1/2 cups extra-virgin olive oil
6 large eggs
Peel potatoes and halve them lengthwise. Thinly slice them crosswise. Pat potatoes dry with a dish towel or paper towels. Toss potatoes with onion, 1 1/2 teaspoons salt and 1/2 teaspoon black pepper.
Place a large skillet over medium heat and warm through. Add potatoes and onion, then add oil. (You need enough oil to almost cover the potatoes, so adjust amount according to your pan size.) Cook until potatoes are just tender enough to cut with a fork, 10 to 15 minutes, adjusting heat so vegetables do not burn or take on too much color. Using a slotted spoon, transfer potatoes and onions to a colander set over a bowl and let cool. Reserve 3 1/2 tablespoons oil from the skillet to use for finishing the recipe. (You can reuse the rest of the remaining oil, too; store it in the refrigerator.)
Heat 2 tablespoons of the reserved oil in an oven-safe 8- to 9-inch nonstick skillet over medium-low heat. In a large bowl, whisk eggs and season with 1/4 teaspoon salt and 1/4 teaspoon pepper. Stir potato mixture into eggs. Pour mixture into pan and use a spatula to flatten out surface. Cook, using spatula to occasionally loosen eggs from edges of pan, until top is almost set, with just a small amount of liquid remaining, about 5 minutes. Flip tortilla onto plate, slide back into pan and cook in the pan for 3-4 minutes. Turn the heat off and let the tortilla sit in the pan for 2 minutes.
You can eat the tortilla right away, or chill it and have it for lunch the next day as wedge on a baguette. A tortilla hits its peak an hour or so after cooking. (Sarah Jay, Fine Cooking)
Serve with Sofrito