In a pan bring the potatoes to boiling and simmer for 10 minutes. Then drain, put back in the pan and partially crush with the back of a fork. You want large chunks in there. Season with salt and pepper.

In a large saucepan with a lid, pour in the milk. Lay the fish in and add the onion and bay leaves and season with ground pepper. Bring to the boil, cover and cook for 5 minutes or until the fish is almost cooked. It can be falling apart or still quite firm, so don't worry too much.

Take off the heat and remove the fish with a slotted spoon and transfer to a ceramic ovenproof dish. Break up the fish a little so that the two types become mixed.

Pre heat the oven to 200 fan, 220 electric or Gas 7.

With a sieve held over a jug, strain the milk that the fish cooked in. Set aside.

In a saucepan over a medium heat, melt the butter (or heat the olive oil) until starting to bubble. Add the flour and stir for a minute.

Take off the heat and add a little of the reserved fish/milk at a time, stirring vigorously to take out lumps, and keeping it a smooth paste. Add more and more, stirring continuously until you've used it all to make the sauce.

Beat in the mustard and white wine. Season with salt and pepper.

Pour over the fish in the ceramic dish.

Arrange the crushed potato over the top. Brush with melted butter or olive oil and bake for 25 minutes until golden brown.