Heat the oven to 200C/ fan 180C/ gas 6. Wrap the potato in foil, place on a baking tray and roast in the oven for 1 hour 15 mins. Wrap the fish in a piece of foil, put on the same tray and continue cooking for another 10 mins or until opaque and cooked through.
Meanwhile, bring a pan of water to the boil, place a steamer on top and steam the cauliflower for around 8 mins or until tender.
Once cooked, halve the potato and scoop out the filling. Flake the fish, removing any bones and discarding the skin.
Blitz the cauliflower, potato and haddock, adding a splash of milk or knob of butter if you like, to loosen the purée.