Flash Flash is one of the best "tortillerías" I have ever been to. The decoration is modern and retro at the same time, since the modern furniture is combined with some elements (like the waiters' outfit) reminding me a lot the aesthetic of Spanish restaurants in the 60s and 70s.
The food is really good. They have a menu, and even though it is not especially cheap (17€), it is very good with abundant portions. When I went with some friends, the meal was a Spanish omelet as a starter, hamburger and fries as a main dish and for dessert a cheesecake with strawberry sauce.