Put the macaroni into a large saucepan of boiling salted water and cook until al dente. Drain and set aside. Pour the milk and cream into the pan with the haddock fillet, bring to a simmer and cook for 6-7 minutes until cooked through.
Remove haddock from pan, and bring milk and cream to the boil, until reduced by a third. Add the mustard and 25g of cheese, and stir until melted. Season to taste with salt and white pepper. Add the cooked macaroni to the sauce and haddock broken into flakes, stir to combine.
Transfer to an ovenproof dish, sprinkle with the remaining cheese and grill for 5-7 minutes, until cheese is browned and the sauce is bubbling.
Serve in individual dishes with a sprinkle of chopped chives.
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