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Tip the dried mushrooms into a heatproof bowl, pour over hot water from the kettle until just covered, then set aside to soak until cool. Meanwhile, heat about a third of the butter in a frying pan until sizzling and scatter in the chestnut mushrooms. Season and fry for 4-5 mins over a high heat until soft and cooked through. Set aside. When the dried mushrooms are cool, drain over a bowl to catch the soaking juices, pressing to squeeze out all the liquid, then roughly chop.
Heat all but about ½ tsp of the remaining butter in a large saucepan over a high heat until sizzling, then add the rehydrated mushrooms and fry for 2-3 mins until well browned. Scatter over the flour and stir together to make a sandy paste. Cook for 2 mins, then gradually pour in the reserved mushroom soaking liquid, leaving any sediment in the bottom of the bowl. Simmer until thickened, then gradually stir or whisk in the milk until completely incorporated into a sauce. Add the mustard powder or mustard and season to taste with a grating of nutmeg and some salt and pepper. Leave the sauce to cook gently on a low-medium heat. Meanwhile, cook the macaroni in a saucepan of boiling water for 2 mins less than stated on the packet.
When the macaroni is ready, drain well and tip back into the saucepan. Pour over the mushroom sauce, tip in the fried chestnut mushrooms, all the cheddar and half the parmesan, then vigorously stir everything together until fully combined. Use the last of the butter to butter a casserole dish, then scrape in the macaroni and mushroom mix. Scatter with the remaining parmesan and the breadcrumbs, if using. Will keep covered and chilled for up to two days or in the freezer for up to three months. Defrost fully in the fridge before baking.
When you’re ready to cook, heat the oven to 200C/180C fan/ gas 6. Bake for 20-25 mins until golden and bubbling. Leave to stand for a few minutes, then bring to the table and serve straight from the dish.