“The best stuffed mushroom in Madrid,” or so I’m told, is in a cramped restaurant on a side street behind one of the city’s main tourist traps, Plaza Mayor.Mesón del Champiñon is not what you’d expect. Creepy, iron-cast devils peer down from light fixtures. Brain-shaped clumps of plaster — they’re apparently supposed to look like mushrooms — have been slapped onto the ceilings.