We got a crave for a Tortilla Española today after finding that our favorite Argentine-accented coffee shop had run out, so came home and fashioned one from an old Victor de la Serna recipe for dinner. Tasted great, although it wasn't as thick as the traditional model because I cut the recipe by one-fourth but didn't use a smaller pan. A minor quibble. Pictures of the frying potatoes and the finished tortilla follow; my recipe, only slightly evolved from Victor's, below them.
Tortilla Española
INGREDIENTS: (Serves two)
3 small baking potatoes or 6 small new potatoes
1/4 cup olive oil
1/4 cup vegetable oil
1/2 sweet yellow or white onion
2 cloves garlic
4 eggs
Water
Salt
Black pepper
PROCEDURE:
1. Peel the potatoes, halve or quarter them if they're large, and slice them thin, 1/8-inch or less.
2. Chop the onion fine and mince the garlic. Break the eggs into a bowl, add 4 tablespoons water, and mix with a fork, adding salt and pepper to taste.
3. Put the oils in a skillet or sautee pan over medium heat. When they're hot enough that a piece of potato dropped in instantly sizzles, put in all the potatoes, stir to separate, and fry them gently. You don't want them brown and crunchy, so watch the heat and try to keep them sizzling without browning. It should take about 8 to 12 minutes to cook them through. Lift out the potatoes and let them drain on paper towels.
4. Using just a bit of the oil, cook the onions and garlic over medium-low heat. Let them cool a bit. Then stir the potatoes, onions and garlic into the eggs. Try to avoid breaking up the potatoes, but don't be obsessive about it; it doesn't matter if a few break or mash.
5. Put the omelet pan back over medium-high heat with a little more of the oil , and pour the egg and potato mix in all at once, spreading it gently with the back of a wooden spoon. Leave on a medium-high flame for a minute or so until the bottom sets; then reduce heat to medium low and let it cook, uncovered, for 8 to 10 minutes, by which time you should notice that the eggs have pretty much cooked through, although they may still be soft at the center. Jiggle the pan, coaxing the tortilla gently with a spatula if necessary, to make sure it will slide free.
6. Now, taking care not to burn yourself, put a large plate over the top of the pan and, holding pan and plate together firmly, flip the pan over so the tortilla will fall onto the plate, cooked side up. Put the pan back on the fire and slide the inverted tortilla back into the pan to cook on the other side for another 5 minutes or so. (If it becomes a little malformed in the flipping process, don't worry, just nudge it gently back into a neat circle.)
7. Slide the tortilla onto a large serving plate and cut it into wedges.
- it seems to me that having the potatoes swimming in a gentle bath of simmering oil makes a difference. It would really be best to use all olive oil, I think. 
Tortillas really need waxy "boiling" potatoes, though. because they're less likely to fall apart; and that's what this is. 