Preheat the oven to 200°C/400°C/Gas mark 6
Melt the butter in a saucepan and add the flour to make a roux. Don’t let it burn, but do cook it over a gentle heat for at least 3 minutes to cook out the flour. It will look like dough. Gradually whisk in the milk. Lumps will come and go, but a whisk sorts this out.
Add the Dijon mustard and white wine. Check the seasoning and add in the parsley. The smoked fish will make it pretty salty, but make sure your béchamel still has good flavour.
Add in your fish and mix well. The sauce will still be pretty hot, so you’ll be able to mix the fish in very easily. Set aside for a few minutes while you finish off the spuds.
Chop the spuds into small chunks and put in a large saucepan of cold water. Bring up to the boil and cook until tender. Drain and put back in the pot with a tea towel on top. This helps them to go light and fluffy when mashed. Add the butter and mash quickly and roughly, then mix in the cheese. Season very lightly.
Pour the fish mix into a large gratin dish and top with a layer of mashed spuds. Bake for about 45 minutes, until the top is golden brown, then allow to settle for 10 minutes. Serve with peas.