There are two "Taberna de los Mundos" in Bilbao: this one, and one in c) Lutxana, near El Corte Inglés. This one is smaller, and I think it's the original one. In any case, you will get the same things in both of them.
La Taberna de los Mundos is famous, among other things, for its sandwiches, small baguettes filled with all kinds of things in strange-looking but extremely well-tasting combinations. The menu changes from time to time, but my favorite is the baguette filled with serrano ham and goat cheese. Amazingly good. You can also ask for croquettes, rabas (fried pieces of squid) or salads, if you prefer.
La Taberna... also has a Menú del Día (a two-dishes lunch menu) which normally offers three or four options for each dish and normally includes some more experimental options altogether with other more "traditional" ones. It's a good alternative for everyday eating, but I wouldn't recommend it for a celebration or a nice relaxed meal, for instance.
In both Tabernas, the atmosphere is quite nice: wooden decoration, photography exhibitions, newspapers and magazines at your disposal... The one in c) Lutxana can get a bit too loud when crowded, though, so if you can choose I would recommend the one in Simón Bolivar better...
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