My housemate is spanish and predictably her spanish omlette is divine. She cooks the potatoes off in the pan in large chunks first, must take a fair while, with the onions and then adds the egg. her's is really dense with potato chunks and she cuts it up into squares. She flips in to cook both sides.
very similar to my tortilla, learnt it from a woman I lived with in Spain, parboil the spuds in big chunks, drain, dry, then sautee in a lot of hot olive oil...then add onions (a lot of Spaniards think onions are WRONG in tortilla)....get the spuds browned, and onion soft...
In a bowl, half a dozen eggs or so...whisk, salt and pepper...
Ideally let the potato/onion mix cool for half an hour or so then dump it into the bowl with the eggs...
get the large frying pan REALLY hot again, with a splash of olive oil, add the mixture, turn pan to minimum and cook for ages, until it's not far from cooked through...
Turn it over by sliding onto a plate and then turning it over into the pan...flip it if it's a small 2 egg one, or if its a massive 8-10 egger it's prob best to finish the top under the grill.
It should be dense, more potatoey than eggy.
Every Spaniard makes tortilla according to their mother's recipe, and any other way is wrong.