Never onions in a Tortilla FFS!
Most spanish use frozen chips part fried (not oven chips), Cilantro and a LOT of eggs.
No they don’t, or at least I’ve never seen one like that in my 17 years here.
The Delia recipe is pretty good, although it doesn’t use anything like enough oil. 1 tablespoon 😯
FWIW my (Spanish) wife’s recipe:
One chopped onion
4-5 potatoes
4-5 eggs
Salt
Lots of olive oil.
Beat the eggs a bit and pour into a large bowl. Add a fair amount of salt.
Deep fry the onion and potato until soft in a large frying pan. They should NOT be caramelized or crispy.
Use a slotted spoon to move the onions+potato into the bowl with the eggs in it. This will naturally include a lot of the oil.
Pour the remaining oil away somewhere.
Pour the egg/potato/onion mix into the frying pan and fry on a medium heat until the centre is only a bit runny. Use a wooden spatula to fold the edges over a bit.
Put a large plate over the frying pan, invert, then slide the omelette back into the pan.
Fry for a while longer until the bottom is set.
How runny the middle bit should be is a matter of debate, and depends on taste. IMO it ought to be a bit runny, but not liquid. My wife likes it a bit more done, and as she makes them…
Edit: This is a Madrid / Extremadura omelette, other regions may include other stuff. Still never seen cilantro in one though!