Heat oven to 200C/fan 180C/gas 6.
tip the mushrooms into a bowl and pour
over boiling water to just cover them. Leave
to cool, then drain (reserve the stock for
another time), squeezing out excess liquid.
Heat about a fifth of the butter in a pan and
stew the garlic over a low heat for 3-4 mins.
Throw in the mushrooms and continue to
cook for 15-20 mins until tender. Season
well and set aside.
Peel and slice the potatoes (don't wash them, as the starch holds the cake together). Butter a 20cm
ovenproof pan, then overlap a layer of
potatoes on the bottom. Build up the cake with
layers of potato, scatterings of mushroom,
dots of butter and seasoning, finishing with
a layer of potatoes. Cover with foil and bake
for 50-60 mins, until a knife slides into the
cake easily. Remove from the oven, run a
knife around the outside to make sure it
doesn't stick, then leave to relax for at least
5 mins. Carefully invert onto a baking tray, cool
and keep in the fridge. To serve, heat in the
oven until piping hot and cut into wedges.