Method
Pre-heat oven to 190C.
Gently cook the onion in 40 grams of the butter until soft, then stir in the flour and mix well.
Gradually whisk or stir in the white wine and fish stock, bring to the boil, season lightly with salt and pepper and simmer gently for 15 minutes. Give it an occasional stir until the sauce has reduced and thickened slightly.
Add the mustard and cream, simmer for another 5 minutes.
Add the fish and parsley, simmer for 5 minutes and season with salt and pepper again if necessary.
Transfer the mixture to 4 individual ovenproof dishes – or one large one – and leave to cool a little.
Meanwhile, boil the potatoes until tender, then drain well and mash with the remaining butter and some seasoning.
Use a piping bag or a fork to cover the fish mixture with the mashed potato; you will find it easier if the potato is still warm. Scatter with the breadcrumbs and parmesan.
Bake in the oven for 15–20 minutes, until lightly browned on top.