Not at all like the Mexican tortillas we know so well, a Spanish tortilla is often compared to a frittata or omelet. But, as the directions reveal, it's different.
Spread a layer of potato slices in a medium saucepan or skillet, followed by a layer of onions. Sprinkle with a little salt. Continue layers until potato and onion are used. Pour olive oil over vegetables, pressing gently to make sure vegetables are submerged.
Clean pan. Using remaining potato-onion mixture, cook a second tortilla the same way.***
* Use a mandoline, the planer on a box grater or the slicer attachment on your food processor, or a cook’s knife.
** Alternatively, use an 8-inch skillet to cook the entire mixture.
***Refrigerate the remaining oil in a jar to use for seasoning potatoes and other vegetables.