
One of the finest delights in life
In Spain, you can never please everyone when it comes to including onion in tortilla de patatas.
So in bars they make them according to their customer's tastes, usually having both. You can find them in supermarkets, precooked, frozen, in jars just ready to mix with eggs...always in both styles.
No discussion possible about olive oil - it's a must. The better the olive oil you fry the potatoes in, the better flavor. Just ask the people who have lived here how chips fried in olive oil by traditional ways taste - it's just completely different.
Besides that, tortilla de patatas should be consumed warm (no need to burn your mouth), and there is also a big controversy about whether to leave it juicy (not completely cooking the egg inside), or well done. The best compromise is making it about two inches thick a bit juicy in the middle, but not everyone likes eggs that way. Like paella, once you have tried a really really good one (everyone boasts they can reach perfection, but not so many are the chosen ones), you will always remember THAT taste.
I prefer the onion variety, a 5:1 potato / onion ratio makes a good balance; between 2-3 inches thick, golden - dark brown outside, rare inside. Thin layers of potatoes fried in extra virgin olive oil with a pinch of salt, and ready to mix with the eggs just before they start to get golden. I particularly whip in an extra yolk for every five eggs.
And just for the fun, there are varieties in the north, especially for the winter, where they include bits of green & red peppers, courgettes and even eggplant/aurbergine that have been previously fried a bit. Fantastic with bread & Rioja or any kind of red wine. And another famous variety is the one with chorizo (red sausage) - which gives it a stronger flavor than bacon, and an overall a red colour. It really drives the winter away. Bacon is nice in tortilla, but it doesn't have that "extra" punch.
And last, a usual tortilla de patatas with a couple of pinches of real saffron makes another delight.
It should be World Heritage.