No, not the tortilla that you wrap a burrito in, this is the Spanish Tortilla de Patatas, or potato and onion omelette. It's a popular tapas item but works just as well as a main or starter, warm or cold. On our way back from Barcelona in 2010, we even had it served as a sandwich ingredient on a baguette.
Potatoes - 500g thinly sliced
Onions - 1 medium, thinly sliced
Eggs - 4 large
Olive Oil - 1/2 cup
Salt
Slice up the potatoes and put them into a large bowl (that you will use again later) and sprinkle plenty of salt over them and mix it through.
In a large frying pan heat about ½ cm of olive oil. When it's very hot add the potatoes from the bowl and start to fry them. Be sure to keep stirring them so that they don't stick or start to brown.
After about 5 minutes add the onion to the potatoes. Stir the contents and keep checking the frying pan to be sure that the potatoes aren't turning brown. Turn down the heat if necessary.
Now beat the eggs into a bowl and add a pinch of salt and pepper and beat them.
Once the potatoes break easily under the touch of the stirring spoon they are ready. So now add the potato/onion mix to the bowl of beaten eggs. Drain away any excess oil left in the frying pan oil at this point.
Mix the potato and egg mixtures together well whilst the frying pan gets very hot with no extra oil in it.
Now add the mix to the frying pan. Flatten it down in the pan and keep the heat at medium for a few minutes.
Put a plate over the mix and turn over the tortilla gently. When you return it to the pan press down the sides to create the classic shape of tortilla de patatas.
Turn the tortilla several times. You'll find it gets heavier each turn.
It is ready when you put a knife into it and the knife comes out clean.
There seems to be a couple of key points to the process. The potato and onion mixture should be cooked in the oil, but not fried. They should not be allowed to brown, you are not looking for that flavour. The warm potato mixture is added to the beaten eggs and let sit for a bit to thicken up. Good luck with the flipping.
Spanish-Fiesta - The base version
XMission - slightly different take