Our best food experience in madrid was coming across the Meson del Champinon which was one of the resturants down the side of the Paza Mayor. We just stopped in for a drink as it looked cute and got to experience a great evening.
The speciality of the house was a plate full (about 10 or so) of mushrooms cooked with oil herbs and chorizo and seasonings and were fantastic, at a cost of about 7 Euros. We actually went back later in the night for another plate and the place was full, with the addition of a keyboard player out the back for entertainment. The decor has to be seen to be appreciated !! There were stone mushrooms on the ceiling and lots of painted brown raised stones decoration on the walls. But we had a great night - at the end of the barman gave me a wee Sangria jug as a momento! (this was also one of the best sangria places we tried too).

Fantastico!!