Pour the boiling water over the mushrooms, cover and leave them to soften.
Put the cream in the pan with the garlic, onion (or shallots), caraway, bay leaf, all spice and peppercorns. Heat slowly, stirring and taking care not to boil the cream over. Cover and leave to infuse for at least an hour, the drain through a sieve and stir the gin though the cream. Taste the cream and season it well.
At this point, I didn't want to discard the left over bits from the infused cream, so I whipped them up in a food processor to layer through the gratin.
Drain the wild mushrooms, snip them into smaller pieces with scissors and add them to your infused cream.
Grate your Parmigiano-Reggiano and slice both the potatoes and the celeriac super fine. You can do it by hand with a bit of pateince, but I used the slicer on my cheese grater for ease. Careful of your fingers!
Preheat the oven to 170C. Layer the potatoes, mushrooms and celeriac into a ceramic dish, along with a little of the blitzed onions and cheese, seasoning as you go.
Pour over the infused cream, cover in foil and cook in the oven for an hour and a half.
Remove the foil, turn the oven up to 200C and pop back in for another 15 mins until the top is golden.
Once it's ready, let it sit for 30 minutes. The potatoes will soak up the liquid, making for a delicious, juicy gratin.