Bar Via 71 is an old school bar in the sense of restaurant/cafe/bar supercombo (not cocktail bar). Inside the walls are covered in shelves of beer, football flags, and old framed photos.
They are famous for their sandwiches - simple but excellent. Think crusty bread, butter, Spanish ham and cheese. That's it, it's all you need.
I'm a fan of their tortilla española, get it "bien babe" - pronounced bah-beh - for a runnier egg tortilla, as seen in the photos.
To really fit in with the locals, read a physical newspaper as you sip you coffee.