Apparently this type of sandwich was made for the posh folk when they went out on shooting (or hunting) trips as it travels well and probably improved even more after being squashed in someone's saddle bag! Alas I have no shooting trips planned but have got the week off work so what better time to make it?!
Be warned - although this is a very simple sandwich, it does take a while to prep and is best left pressing for at least 5 hours!
To begin with you need to make the mushroom mixture - I used half chestnut mushrooms and half white closed cup mushrooms, about 14 medium sized mushrooms in total.
Finely dice the mushrooms.