Cooking a True Spanish Potato Omelet ("Tortilla de Patatas" or "Tortilla Española

Here's a recipe (Jim has made slight modifications) from Enrique Pareja, who loves Tortilla as much as we do

8 eggs
2 lbs potatoes (actually 2 1/4 pounds...I'm translating from metric)
1 large onion (optional - preferably yellow)
2 cups oil (some will be left over. Use a light grade olive oil (not virgin) for best "authentic" results)
Salt (to taste)
Peel the potatoes, cut them in half lengthwise, and cut them in thin slices (a bit less than 1/4 inch thick). Rinse the slices in cold water and dry them VERY WELL with a clean towel, one by one if necessary. Dice the onion into fairly small pieces.
Heat the oil in a skillet (non-stick is best) and add the potatoes, onion, and a little salt.
Stir occasionally with a spatula or slotted spoon. The heat should be medium low - so that the potatoes cook through without getting brown on the outside.
When the potatoes are cooked through, remove from heat and transfer them to a large colander to let them drain. (The oil can be saved and used again.) Leave a little bit of oil in the skillet, just enough to cover the bottom.
In a large bowl, beat the eggs well with a fork, add a little salt, and add the (now drained) potato/onion mixture. Stir well, and let stand for a few seconds.
Heat the small amount of oil left in the skillet over high heat; when it starts smoking pour in the egg/potato/onion mixture and shake the skillet a little by the handle so that the mixture is distributed evenly. Turn down the heat, and when you see that the mixture is beginning to set, flip the tortilla (using a large plate over the top of the skillet, and then sliding the tortilla back into the skillet so that the other side can cook)
When it's set (depending on your taste - in any case the eggs should not be totally cooked, so that it's a bit "juicy" inside *) slide it onto a large plate, and serve. You may also serve it at room temperature.
"Bon Apetit!"

* --Enrique is from Madrid, and they prefer their tortilla pretty runny there. It's not necessary, you can cook it more. Also, Madrid people cut their potatoes in slices, while small chunks are also ok (it's always done that way in Galicia).

Note for the uninitiated
This is like a fritata - it's served like a cake, in wedges or squares. The thickness will depend on the size of your skillet and your taste.
If you are very worried about the flipping, you may put the skillet under the broiler for a few minutes to brown the top, and then flip the finished tortilla out onto a plate, (since the more atractive side will be the one first cooked in the skillet.) I've also found that the rinsing and drying of the potatoes is not absolutely essential if you're in a hurry and/or very hungry! Also, at the risk of committing blasphemy, it is very good with added twists - maybe a little parsley or cheese thrown in to the egg mixture before cooking - whatever suits your fancy once you get the hang of it. But of course, you must NEVER serve it with mexican salsa!!