Potato Gratin with Porcini

Place the dried porcini in a heatproof bowl, pour over 250ml boiling water, and leave to soak for 30 minutes. Lift the porcini out, squeeze gently, and chop roughly. Pour the soaking liquid through a fine sieve lined with kitchen paper, reserving it carefully — discard any grit at the bottom.

While the porcini soak, peel and slice the potatoes to 3mm on a mandoline or with a sharp knife. Keep the slices submerged in cold water to prevent browning. Grate both cheeses, dice the shallots, mince the garlic, and measure out the cream, milk, and wine.

Melt the 20g butter in a wide saucepan over medium heat. Add the shallots and cook, stirring frequently, for 5–6 minutes until soft and translucent but not coloured.

Add the minced garlic and chopped porcini to the pan and cook, stirring, for 2 minutes until fragrant.

Pour in the white wine and cook over medium-high heat for 3–4 minutes until the wine has almost completely evaporated.

Pour in the reserved porcini soaking liquid and cook over medium-high heat for 4–5 minutes until reduced by half and syrupy.

Add the double cream, whole milk, thyme sprig, bay leaf, salt, pepper, and nutmeg. Stir to combine, then bring the mixture just to a simmer over medium heat — look for small, lazy bubbles at the edges.

Drain and dry the potato slices thoroughly with a clean towel. Add them to the cream mixture and stir gently to coat every slice. Remove from the heat and discard the thyme sprig and bay leaf.

Preheat the oven to 180°C, or 160°C for a fan oven, or gas mark 4. Rub a 30 × 20cm gratin dish thoroughly with the cut side of the halved garlic clove, then butter the dish generously.

Transfer one third of the potato slices into the dish, overlapping them in neat layers. Spoon over one third of the cream and porcini mixture, then scatter with one third of the Gruyère. Repeat for two more layers, finishing with the remaining cream poured evenly over the top.

Mix the remaining Gruyère with all of the Parmesan and scatter evenly over the surface.

Cover the dish tightly with foil and bake for 45 minutes, until a knife slides through the potatoes with no resistance.

Remove the foil and bake for a further 25–30 minutes until the top is deeply golden and bubbling at the edges.

Rest the gratin uncovered for 10 minutes before serving — this allows the cream to set and the layers to hold when portioned.