A Spanish tortilla or Spanish omelet is not like the round corn things we were wrapping fajita fillings with last week; it's potatoes and eggs. I believe the two dishes have the same name because they have the same shape (little round cakes).
Like many basic foods, people get really worked up about the proper technique for making these. In fact, while listening to podcasts while making a tortilla, I heard Amanda Hesser from Food52.com mention how upset people got when she published a non-traditional approach to making tortillas (it was an interesting podcast overall on vegan cooking, linked here - this is not a vegan food, don't get confused).
Hesser noted that there's a difference between honoring tradition and stifling creativity. Which makes sense and her Spanish-ish tortilla recipe is here. I didn't follow it, mind you, I went my own way and cut potatoes thinner, then baked them at high temp to get them nice and crispy before going into the eggs. The goal is to have a crispy tortilla to put in the soft roll and mayonnaise (which is doctored with just a splash of olive juice to make dirty mayo).
The final sandwich is delicious.
One note - it also changes the reheating instructions. I strongly recommend cooking the tortilla square in a pan with nice hot olive oil so it's not just warm, but also crisped on the outside, then putting in the roll with mayo on top and bottom. I realize that it's weird to make eggs a little crispy, but that's what we're going for here. I'd also recommend having a cold beer nearby because you'll want one.