Swedish fish gratäng (Swedish Fish Pie)

My very Swedish mother has had, and always will have a vendetta against all ready meals, she would swiftly walk past the frozen meal section uttering expletives under her breath, but bizarrely despite her huge hatred for all things pre-packaged she had a weakness, a chink in her scratch cooked armour.... Fisk Gratang, not just any Fisk Gratang, FINDUS fisk gratang. I know exactly what you are thinking, and believe me I questioned her motives many times, but even I have to say it is the one ready meal that is absolutely delicious. 
 Like I have mentioned before, as a child my mother would use me as her Swedish food mule, her suitcase would be filled with our clothes and my suitcase would be filled to the brim with Swedish food, you name it we had it, and believe me we always and I mean ALWAYS had fisk Gratang. So you can only imagine my mothers joy when in the late 90's her Swedish friend called to alert her that Findus had started to sell fisk Grantang in Sainsbury's supermarket, my mother hot wheeled the car so fast and literally filled up a trolley with every pack of the fisk gratang that the freezer could hold, she even asked if they had any more in the back. 
 Sadly the Fisk Gratang gravy train had to stop sometime, and despite my mothers efforts to double Findus's sales, narrowly bankrupting my father in the process, they stopped selling it, and my mother went back to muttering expletives in the frozen ready meal section. This is why I have spent a long time trying to re-create the exact Fisk Gratang just how my mother remembers it.
 This is a version of a fish pie, but it is much much lighter despite the copious amounts of double cream, it is easier to make than a British fish pie, and I'm sorry I have to say it, it is far far better. 
 Once you try this Swedish Fish Gratin I fear that you may actually turn your back on the humble British fish Pie, so don't Say I didn't tell you so!!! I recently cooked this recipe for my mother, I think I saw a tear in her eye when she tasted it, job well done I think.

For the fish stock:
The prawn peelings from the 200g of north atlantic cooked prawns
1 tsp salt
187ml bottle white wine
300ml fish stock (water with two know fish stock pots)
A few fennel sprigs
1large shallot
10 white pepper corns

To finish the sauce:

300ml of the fish stock that we made earlier
50g butter, flour
200ml milk
225ml double cream
3/4 tsp salt
25g chopped dill
2tsp mustard
20 ml lemon juice
1/4 tsp lemon zest
1 tsp mild paprika

Duchess potatoes:
3 very large potatoes
150g of butter
100ml of milk
100ml of double cream

1 tsp Maldon sea salt

4 egg yolks

1 tsp ground white pepper

1/2 tsp of freshly grated nutmeg

For the Filling:
500g cod loin
200g North atlantic prawns with the peel on

 Peel and cube the potatoes and boil them in well salted water until they are very tender.
 Meanwhile place all the stock items into a pot and reduce it down by a third, it is important that the peelings from the prawns including the heads are in the stock pot. Strain the stock through a sieve, you should have roughly 300ml of stock.
 In another pan start the sauce by melting the butter and flour together to create a rue, add the stock a little at a time, whisking as you go, it is just like making a white sauce, once the stock has been used up then add the milk and double cream, when the sauce is smooth add all of the other ingredients for the sauce, mustard, dill, lemon juice and zest, salt and paprika(for colour), whisk until smooth, set to one side.

 Next pass the potatoes through a potato ricer or mash them as finely as possible, place a pan on the heat and add the butter, milk and cream, then the potatoes, then the egg yolks one at a time, continue to mash or stir until the potatoes almost seem gluey. Finish with salt, pepper and the nutmeg. Place the potatoes into a large food piping bag with a large nozzle ready to decorate the pie.
 Butter a Medium sized pie dish, roughly 15cm by 20cm. Slice the Cod loin into inch thick slices and place the fish on one level, overlapping slightly, season with a little salt, then top with the peeled prawns, then the sauce.
Pipe the potato around the edges, making sure that a space in the middle is left uncovered.
Bake in the oven for 25-30 minutes at 180 degrees c until golden and bubbling.