Pour the milk into a saucepan and add the bay leaves, onion, white peppercorns and thyme.
Bring slowly to simmering point.
Remove from the heat and leave to infuse for as long as possible.
Strain the infused milk into a jug.
Gently melt the butter in a heavybased saucepan.
Add the flour and cook, stirring, over a low heat for 1 minute, without browning. Gradually add the infused milk, stirring constantly with a wooden spoon.
Make sure the mixture is smooth and well blended each time before you make the next addition.
When all the milk has been added, continue stirring until you have a smooth, silky cream.
Bring to the boil and simmer on a low heat for 2 minutes; this helps round off the flavour.
Season the sauce to taste with salt and white pepper and use a Microplane to add a pinch of fresh nutmeg
Remove from the heat. Preheat the oven to 180°C/Gas Mark 4. Cut the fish into large chunks.
Put them into a gratin dish in a single layer.
Season the fish with salt and pepper, pour over the melted butter and fish stock.
Cover the dish with foil.
Place in the oven and bake for about 15 minutes, until the fish is only just cooked. Remove from the oven and leave to rest for a few minutes.
To make the crust, melt the butter in a frying pan.
Add the onion and cook gently until softened.
Put the fried onion into a bowl with all the remaining ingredients, mix well and season to taste with salt and pepper.
Pour off all the juices from the gratin dish containing the fish and set aside.
Gently reheat the béchamel sauce, if necessary, then add 4–6 tablespoons of the fishy juices to it – but be careful not to thin down the sauce too much.
Stir in the parsley and crème fraîche and season to taste.
Add the prawns to the fish.
Pour the sauce over the top.
Scatter over the crust. Place in the oven and bake for about 25 minutes, until browned and bubbling.