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.
I don't think it matters whether you use onions or shallots - shallots are a little more fiddly to peel and chop but I like the flavour. I finely sliced 8 shallots and then softened them in a little butter and olive oil then added some very finely sliced garlic (3 cloves).
Once the shallots have softened, add the chopped mushrooms and cook the mixture down until and excess liquid has cooked away. If there is too much moisture left in the mushrooms the sandwich will become too soggy. Season with salt and pepper to taste. I found that leaving the mushroom mix to stand allows any excess moisture to drain plus it allows you to get on with the rest of the steps!
Slice a "lid" off the top of the bread and scoop out most of the bread inside. If you get a little carried away with the scooping and feel there are any weak spots, you can patch it up with some of the bread you've taken out.