Photo: Laura Edwards
Put the haddock and salmon in a lidded frying pan, pour over 500ml milk and add the bay leaf. Cover with a lid. Slowly bring to the boil, then remove from the heat and strain through a sieve, reserving the milk. Set aside the fish – it should be undercooked.
Rinse the frying pan, return to the heat, melt 25g butter, add the onion and fennel and cook gently for 10 minutes until soft, then stir in 2 of the chopped garlic cloves and cook for 2 minutes. Increase the heat, pour in the fizz or wine and bubble away for 5 minutes until reduced. Stir in the cream and set aside.
Peel and thinly slice the potatoes about 3mm thick (or as thin as you can). Transfer to a medium saucepan with the remaining 400ml milk and garlic and season well. Cover with a lid and slowly bring to the boil. Simmer for 10-15 minutes until tender, stirring occasionally to make sure the potatoes don’t stick to the base of the pan. Strain and discard the milk. Spread the potato slices on a tray to cool slightly.
Preheat the oven to 220°C, fan 200°C, gas 7. Melt 50g butter in a medium pan, stir in the flour to make a paste. Cook, stirring, for 2 minutes, then slowly whisk in the infused milk until combined and smooth. Simmer for 1-2 minutes, stirring until thickened, then stir in the cream, onion mixture and the herbs. Season to taste – be cautious with the salt as smoked fish is quite salty already.
Using a fork, flake the haddock and salmon into a large baking dish of about 2 litre capacity – keep the flakes quite big. Scatter over the prawns and smoked salmon.
Pour over the sauce and stir to combine. Then add a single layer of sliced potatoes in overlapping layers. Sprinkle over half of the parmesan and season well. Top with the remaining potatoes and parmesan, and season. Dot with the remaining butter.
Cover the dish with buttered foil and bake in the oven for 25 minutes. Remove the foil and bake for a further 10-15 minutes until bubbling around the edges and golden on top. Serve straight away.
Get ahead: assemble the fish pie a few hours ahead, cool, cover and chill. To bake from chilled, add 15 minutes onto the cooking time until piping hot throughout. To freeze ahead, assemble the fish pie up to the end of step 6 up to 2 weeks ahead, leave to cool completely then cover in buttered foil. Wrap in a double layer of clingfilm and freeze. Bake from frozen in a preheated oven for 1 hr and 30 mins at 220°C, fan 200°C, gas 7 – removing the foil halfway through – or until piping hot.