With a recipe that contains little more than eggs, potatoes, and onions, you will find many different techniques applied to the preparation of each ingredient. Some call for shallow frying the potatoes in oil and using the oil to cook the tortilla. Some call for cooking the tortilla on the stovetop and flipping it several times while cooking, in order to firm it up and round the edges. I've done this and it produces a great tortilla but not everyone has two pans the same size or, understandably, the will to invert a steaming hot panful of eggs into another hot pan.
For the version I'm sharing here -- which you could call Spanish-ish Tortilla -- I took a nod from Italy and cooked the tortilla like a fritatta (I also added a sprinkling of Parmesan cheese). You can eat it hot, if you like, but it also makes for a great lunch or snack once it's cool. —Amanda Hesser
Place the potatoes in a medium pot. Cover with water, season with about a tablespoon of salt, and bring to a boil. Reduce the heat and simmer until the potatoes are just tender when prodded with a fork, 20 to 25 minutes. Drain and let cool, then cut into 1/8-inch slices and put them in a large mixing bowl.
Heat the oven to 400 degrees. Warm 1 1/2 tablespoons oil in a medium pan over medium heat. Add the onions, season with salt, and cook until translucent and lightly browned, about 10 minutes. Scrape the onions into the bowl with the potatoes.
Lightly whisk the eggs until smooth, then add them to the potatoes and onions, and gently fold everything together. The potatoes should not be sticking together -- make sure the egg separates each slice. Season with salt and pepper and fold once more.
Set a large nonstick or well-seasoned cast iron pan over medium high heat. Add the remaining 1 1/2 tablespoons oil and the butter. When the butter foams, pour in the potato-and-egg mixture and pat it down so it's even on the top. Sprinkle the cheese on top, let the eggs set around the edges, and transfer the pan to the oven. Bake until the eggs are almost set in the middle and the tortilla is browned around the edges, about 20 minutes.
Remove the pan from the oven and let it cool on the stovetop. Run a thin spatula or knife around the edge of the tortilla to loosen it, then invert it onto a plate (this may be a 2-person job). If you'd like to serve the tortilla warm, invert it while it's still warm. Slice and serve. If you'd like to serve it with garlic mayonnaise, I won't argue.
Before starting Food52 with Merrill, I was a food writer and editor at the New York Times. I've written several books, including "Cooking for Mr. Latte" and "The Essential New York Times Cookbook." I played myself in "Julie & Julia" -- hope you didn't blink, or you may have missed the scene! I live in Brooklyn with my husband, Tad, and twins, Walker and Addison.