This recipe combines a few other recipes. We've made this twice now, and continue to tweak the balance; if anything, go lighter on the potatoes, and the cheese may or may not actually work well in conjunction with easy flipping of the tortilla. Nevertheless, it's hearty, tasty, just-oily-enough. Bonus: it's one of those wonderful dishes that's possibly even better as leftovers the next day.

See also http://www.seriouseats.com/recipes/2009/06/dinner-tonight-tortil...

For a tortilla you make in a 9-10" frying pan:

5 small/medium sized boiling potatoes (red, yukon and purple all work)

6-8 farm fresh large eggs, depending on size

1 ~4" link of chorizo (optional)

1/2 cup grated manchego or iberico cheese

Half an onion (medium size), finely chopped

1/4 cup chives and parsely, chopped, optional

Extra Virgin Olive Oil (EVOO)

Salt (about a teaspoon, to taste)

From http://www.finecooking.com/articles/how-to/spanish-tortilla.aspx :

You can make a tortilla with the barest of kitchen equipment, just a bowl and a skillet. The pan must be deep enough to contain all the potatoes and should preferably have gently sloping sides to give the tortilla its shape, which is like a Frisbee. For the recipe that follows, a 10-1/2-inch skillet that's at least 1-1/2 inches deep is ideal. The sliced potatoes will fill the pan, which is fine as long as you turn them carefully as they cook in the oil. Though I'm not usually a fan of nonstick skillets, I do embrace them for tortillas. A tortilla that won't release cleanly from the pan isn't a total disaster, but it is irritating, and messy.

Wash and slice the potatoes, aiming for 1/8 inch thick. Thicker slices take longer to cook and make a dry tortilla.

Fry the potatoes over medium-high heat in a generous amount of EVOO or a 75/25 soy/olive oil blend. Add the salt, onions, and herbs about 5 minutes in.

Slice and sautee the chorizo.

More guidance from http://www.finecooking.com/articles/how-to/spanish-tortilla.aspx:

Once the potatoes and onions are cooked and drained, they're added to the beaten eggs. Some Spaniards let the egg and potato mixture sit for a short time, maybe 15 minutes, so the potatoes absorb some of the eggs. I don't find that necessary, but it's okay to do it, if you want. Other cooks crush the potatoes a bit as they sit in the eggs. That is not okay, in my opinion, as it ruins the layered effect that you get in the finished tortilla.

Here is where I mix in the sauteed chorizo and cheese.

Next step: frying the tortilla, cribbed from Fine Cooking:

The egg, potato, and onion mixture gets cooked in the same pan that you used to fry the potatoes and onions. Here's where a little knowledge goes a long way.

• Wipe out the skillet. If it's not nonstick, use a spatula to scrape out any stuck-on bits, and then wipe out the pan with a wadded paper towel.
• To prevent sticking, heat the skillet on high. In a hot pan, the eggs coagulate immediately, before they have time to fill the tiny pores in the pan and stick to it. It doesn't matter how much oil you add to the pan -- if it isn't hot enough when the eggs go in, the tortilla won't come out in one piece.
• After the mixture cooks for a minute, reduce the heat. This ensures that the inside sets before the outside burns. A low temperature also seems to make the eggs firmer and denser, which is what you want in a tortilla.

Covering the setup also help it cook through and set the top eggs, enabling you to flip it for the final step:

Give the pan a good shake to release the tortilla. If it isn't loose in the pan, help it along with a spatula. The eggs will be a little jiggly and wet in the center, but the tortilla should slide around as a whole unit.

Find a flat plate that's at least as wide as the pan and has no rim. (In Spain, there exists a special plate whose sole purpose is to flip tortillas—what devotion.) To do the flip, you'll invert the tortilla onto the plate and then slide it back into the pan to finish cooking.

Here are the source recipes: http://www.geocities.com/spanishtortilla/ ;
http://www.finecooking.com/articles/how-to/spanish-tortilla.aspx