and having made innumerable tortilla espanolas since returning, my one problem was always the amount of oil you use. Traditionally, you fill the pan with an inch of oil to fully cook the potatoes first, then the onions second. I would swap the cooking of these two ingredients, as the potatoes take much longer. Cook until the are starting to fall apart around the edges and then add the onions. Also, allow potato/onion mixture to cool completely before adding to the eggs to prevent the eggs from cooking early. A tip that I like to use to get a really nice set tortilla is to allow the potato/egg mixture to soak in the refrigerator for 20 to 30 minutes. Flipping is very hard, but getting a plat that completely covers the pan helps, and the plate needs to be flat as well. To prevent sticking, coat the pan (bottom and sides) with a little oil. The great thing about tortillas is you can add ANYTHING to them. Cheese, meats, vegetables .... enjoy!