Restaurante Juan José
It's a bit out of the way (north of the centre) but locals regularly flock to this humble establishment for its fabulously gooey tortilla española (drenched in mayo). But the tuna (fresh from Isla Cristina) and carne mechada (meat stuffed with pepper and ham) are equally amazing. If you arrive after 2pm you'll have to wait for a table. It's located about 1.