1 medium celeriac (about 400 g)
3 medium King Edward potatoes (about 600 g)
A wedge of lemon
400 ml thickened cream
6 sprigs of thyme, plus an extra 2 sprigs of thyme for garnish
1 bay leaf
¼ onion, skin removed
60 g Gruyere, thinly shaved
15 g parmesan, finely grated
30 g Australian Black Truffle, finely shaved (we recommend using Eugenio Brezzi shavers – pick up from Madame Truffles in store or online)
Preheat oven to 180 degrees (fan). Butter a small oven-proof dish.
Peel celeriac and potatoes, and using a mandolin (or a knife, if your skills are good), thinly slice the celeriac and potatoes. Bring a large saucepan of water to boil and add the sliced celeriac. Reduce to a simmer cook for 2 minutes then add the sliced potatoes. Cook for a further 7 minutes, or until they are tender but not falling apart. Drain water and set the potatoes and celeriac aside in a bowl.
Using kitchen string, tie 6 sprigs of thyme and a bay leaf together and place in a medium sized saucepan. Add the cream, peppercorns and quartered onion. Over a medium heat, bring the cream to a simmer. Reduce to low and simmer for 5 minutes. Take off the heat and leave to infuse for a further 10 minutes.
To prepare the gratin, place a layer of potato in the bottom of the buttered oven-proof dish, with the edges just overlapping. Then add a layer of celeriac and, on top of the celeriac, thinly shave 5 g of Black Truffle. Repeat this step until the remaining potato and celeriac is finished. Season the last layer of potate/celeriac with sea salt. On top of the last layer, evenly place the Gruyere slices and scatter finely grated parmesan and the leaves of two sprigs of thyme. Season again with a pinch of sea salt. Cook in oven for 30 minutes; the top should have a golden crust. Remove from oven and thinly shave an extra 10 g truffle on top to serve.