Heat the oil or butter until very hot then fry the mushrooms until they have picked up a little colour & are slightly crispy around the edges. Remove from heat and drain on kitchen paper.
Pop all the ingredients Into a blender, and mix until a coarse or smooth paste, depending on what you fancy.
At this point have a little taste. If it tastes bland, add more anchovies, if too salty, add more capers, and if it is too dry add a little olive oil.