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Blend together all the ingredients for the breadcrumbs in a food processor until roughly chopped, season, then set aside. Boil the macaroni 1 min less than the time on the pack, drain, toss in the olive oil and set aside.
Tip the onion, milk and the smoked haddock into a pan and season. Simmer gently for 10 mins, then remove the haddock. Flake the flesh and drizzle with the truffle oil and set aside.
Mix the butter and flour together to make a paste. Over a low heat, whisk the paste into the hot milk and bring to the boil, stirring all the time. Remove from the heat and add half the grated cheese and stir well.
Take the pan off the heat, tip the pasta into the sauce and mix well, then fold through the haddock and season with ground black pepper. The sauce may seem a little thin, but it’ll thicken as it cooks. Butter a large ovenproof dish and tip in all the macaroni (I like it piled high), then finish by scattering over the rest of the cheese and most of the breadcrumbs. This can be prepared a day ahead, covered and kept in the fridge.
To cook, heat oven to 200C/180C fan/gas 6. Bake for 20 mins until bubbling and sprinkle with the rest of the breadcrumbs. Bake for 5 mins more, then drizzle a little more truffle oil on top, and scatter some extra herbs, if you like.