How to cook fish pie

I could be completely wrong here, but I think if you put raw fish pieces in the base of a pie dish, cover with sauce, top with mashed potato and cook it, then some liquid may find its way out of the fish and you will end up with a pie whose sauce is a little inconsistent.  But then of course, the key to cooking fish well is to not over-cook it.  So in the below recipe, where basically the fish is poached in milk before that milk is turned into your sauce, one trick is to keep the poaching brief.  It also means you don't have to worry about fish stock.

To poach the fish:

Mixed fish pieces
Whole milk to cover
1 bay leaf
4 peppercorns
2 allspice berries
2 cloves

Other pie contents (suggested):

1 leek, halved vertically then sliced and washed
1 carrot, peeled, quartered lengthways, chopped and simmered until tender
Some button mushrooms or quartered ordinary mushrooms.  Sauteed in butter until their liquid is reabsorbed.
Halved hard boiled eggs (Cover eggs with cold water, bring to a simmer, simmer for 4 minutes, leave for a further 3 minutes in the pan then drain and rinse with cold water.)

To make the sauce:

30g of butter and 20g of plain flour per 400ml of milk (see below)
White pepper
Lemon juice

To top:

Stiff mashed potato, melted butter

Preparation:

Assemble the poaching ingredients in a saucepan.  Bring just to a simmer quite quickly then pour into a colander over a bowl.  Pick out anything other than fish from the colander.  Pour the milk from the bowl into a measuring jug so you know how much butter and flour to use, but place the colander back into the bowl to catch any further juices to use as well.

To make the sauce, melt the butter in a saucepan then add the flour.  Whisk over a low heat for 1-2 minutes as it bubbles.  This is called 'cooking out' a roux.  For some recipes, though less so in cookery these days I believe, you aim to keep cooking a roux it until it darkens in colour to the degree desired and becomes nutty in smell.  Here any darkening or nuttyness is to be avoided, so keep the heat low.  Though if you don't cook it enough the sauce will be floury.  Enough said!  Add the milk 1/3 at a time, whisking and heating after each addition until thick.  Once all is added, simmer for 2 minutes and adjust the seasoning with salt, pepper and lemon juice to your taste.

Assemble the pie contents in a nice dish.  Pour over enough sauce to cover then use a spatula or spoon to just make sure the sauce is distributed around and under things.  Use a serving spoon to lay out large blobs of mashed potato on the surface, then use a large fork to flatten them down and into each other, and create a rough surface.  Finally use a spoon to cover some or most of the surface with melted butter.

Bake for 30 minutes at 210C, turning around after 15 minutes if necessary, and rest for 10 minutes before serving.  Enjoy!