Tortilla (pronounced tortiya if you're from northern Spain or tortisha from the south and from S. America. I do the latter), was a regular at our house when I was growing up. I remember my mother making each individual one and we each had to wait in turn. Although sometimes we shared if we were really hungry.
It's such a simple dish, although it takes a while to get the potatoes cooked through. When I made it last night, I realised that it's actually a great frugal, waste-not, and whats-left-in-the-fridge type of meal.
All you need are potatoes, eggs, seasoning. That's the classic version. I add onions, oregano, and a dash of milk to the eggs. But you could also add all types of left-overs, such as thinly sliced peppers, sweetcorn, shredded meat such as roast chicken, ham or salami. It's really as versatile as an omellete, although I suppose it's no longer Spanish tortilla, but who cares right?
Lightly and slowly fry/saute about 2cm cubes of potato in olive oil, then add onions and continue to fry until onions are soft and potatoes are cooked.
Beat eggs, add a small dash of milk, throw the eggs into the frying pan.
If you're making it thick, you can transfer the pan under a grill to finish off the top. Or do it how my family does, slide the tortilla onto a plate, flip the pan over and place over the plate, now flip everything over again so that now the plate is in the pan. Don't try this unless the tortilla is well cooked on the bottom and there's not much runnyness left.