Mushroom and Chestnut Parmentier

Peel and quarter the potatoes. Dice the shallots, mince the garlic, strip the thyme, chop the rosemary, crumble the chestnuts, and grate the Gruyère. Set everything aside in separate bowls.

Place the potatoes in a large pot of cold, well-salted water and bring to a boil over high heat. Cook until completely tender when pierced with a knife, about 20 minutes, then drain thoroughly and allow to steam-dry in the colander for 3 minutes.

Pass the drained potatoes through a ricer or fine sieve back into the warm pot. Fold in the cold butter cube by cube over very low heat until fully absorbed, then stir in the warm milk, egg yolk, half the Gruyère, and a generous grating of nutmeg. Season assertively with salt and pepper, then set aside.

Heat the olive oil and 1 tbsp butter in a large, wide ovenproof skillet or sauté pan over high heat until the butter foam subsides. Add the torn mushrooms in a single layer — do not stir — and roast undisturbed until deeply golden on one side, about 5 minutes.

Stir the mushrooms, reduce heat to medium-high, and cook until any released liquid has fully evaporated and the mushrooms are caramelised all over, about 5 minutes more.

Push the mushrooms to the edges of the pan, add the shallots to the centre, and cook until softened and lightly golden, about 3 minutes.

Add the garlic, thyme, and rosemary to the pan and stir everything together. Cook until fragrant, about 1 minute.

Stir in the tomato paste and cook, pressing it into the base of the pan, until it darkens slightly, about 2 minutes.

Pour in the dry sherry and scrape up all the fond from the base of the pan. Cook until the liquid has reduced by two-thirds and is syrupy, about 2 minutes.

Stir in the soy sauce and the crumbled chestnuts. Season generously with salt and pepper, then remove from the heat. Preheat your oven to 200°C, or 180°C for a fan oven, or gas mark 6.

Transfer the mushroom and chestnut filling to a 28 cm ovenproof baking dish if not already using one, spreading it in an even layer. Spoon the mashed potato over the top and spread gently to the edges, then use a fork to create ridges across the surface. Scatter the remaining Gruyère evenly over the top.

Bake until the topping is deeply golden and the filling is bubbling at the edges, about 30 minutes. Allow to rest for 5 minutes before serving.