Photograph: Tara Fisher
Start by making the béchamel sauce. Melt the butter in a medium pan over a medium heat. Add the flour and beat hard until you have a smooth paste. Take off the heat and leave to cool for 2 minutes, then gradually add the milk, whisking constantly.
Return the pan to a medium heat; add the onion, clove and bay leaf and simmer for 10 minutes, whisking frequently. If the sauce becomes too thick, whisk in a little more milk.
Finish the sauce by removing the onion, clove and bay leaf, then add the nutmeg and season with salt and pepper – I like to use white pepper. Leave to cool slightly.
Preheat the oven to 180°C, fan 160°C, gas 4. Slice the potatoes into ½cm-thick rounds. Cut the haddock into small chunks and add to the béchamel sauce with most of the parsley. Mix together, then add the potatoes.
Transfer to a baking dish, about 18cm x 22cm; top with the grated cheese. Bake for 25-35 minutes, or until bubbling. Sprinkle with the reserved chopped parsley before serving.
Get ahead: assemble to the end of step 5 up to 2 hrs ahead; chill.
This recipe is adapted from The Little Paris Kitchen by Rachel Khoo (Michael Joseph, £20)