Put the venison fillet in the freezer and leave until it is slightly 'stiff' (mine took about an hour). Remove from the freezer and slice as finely as possible, then take each slice and beat out under clingfilm so that you can just about see through the slices. Lay the slices neatly on a serving plate, season with pepper and cover with clingfilm before putting in the fridge
Heat the cooking oil in a large skillet and add the shallot. Cook until it is just caramelising at the edges - that should take about 15 minutes on a low heat.
Clean and trim the mushrooms and slice any larger ones.
Now add the mushrooms and fry on a medium to high heat for about 8 minutes till the mushrooms are cooked through. Make sure you don't stew the mushrooms but cook them on a high enough temperature to fry. Mix the balsamic, virgin olive oil and thyme in a jug and season with salt and pepper to taste. The mixture should taste like a robust salad dressing.
Put the mushrooms into a bowl while still warm and add the dressing. Then leave to cool down for an hour or so.
Serve by dressing the serving plate with whatever green leaves you have and putting a mound of mushroom salad in the centre of the venison.
Provide a little more balsamic and oil dressing for guests to add to the carpaccio to their own taste. You could also add shaved parmesan or pecorino cheese if you think you need more flavour. It depends a bit on the quality of your mushrooms.