Yes I could randomly pick one off the internet but do you have a tried and tested one that is fairly simple but tastes great ? Please?

I have the fish (frozen pollock from freezer) to poach in milk and the potatoes to boil and mash but the sauce is the essential part of a good fish pie.

I don't have a tried and tested one.

I would make a roux with equal amounts of butter and flour and slow add and whisk in the milk that you poached the fish in.

Add some onion to the milk.

I was thinking of something like that but hadn't thought of the onion - yes maybe that would improve it.

Butter, milk, cornflour and parsley is what I use. I poach the fish in it.

and perhaps a little nutmeg or dill?

Sauces may well taste lovely for some but rank to others.

Thats why it is always best to experiment to what is personal to you.

Marco Pierre White did a simple version on a fish pie once:

300ml of double cream

1.5 Knorr fish stock cubes

Granted, not exactly classy, but tastes okay.

Here's what I do, very simple:

Bring 1 pint milk up to boiling point in a saucepan. Turn off as soon as reaches the boil, and put in the fish with a couple of bay leaves. By the time the milk has cooled down, the fish will be perfectly cooked.

Then remove the fish from the milk (I usually pour the milk through a sieve to get all the fishy bits out and the bay leaves, which you can discard) put the milk back in saucepan with 1 oz butter and 1 oz plain flour. Heat, whilst continuously stirring with a whisk or spoon. Once it thickens up, the sauce is ready for flavouring.

I usually add:

Handful of chopped parsley, large handful of grated strong Cheddar cheese, salt and pepper.

Then you're more or less done, put the fish back into the saucepan, with whatever else you're adding, eg cooked prawns, chopped hard boiled eggs etc, mix, taste to check seasoning, then out in dish and top with potato.

You could also put capers in the sauce if you like the, but I don't bother, as I usually make a potato & caper rosti topping instead of mash.

I was thinking the same, only with parsley instead of onion.

The one I use is out of the Oxo cookery book. You poach the fish in water, then take the fish out and put it in the pie dish. Reduce the leftover water to 1/4 pint. Make a white sauce with 1/2 pint milk etc. and add the reduced stock to the sauce before pouring it over the fish.

You're supposed to add a veg Oxo cube to the water but I use Marigold Organic Stock.