Pour 500ml milk into a shallow pan. Peel and chop the onion into 8 pieces, add to the milk with the peppercorns, bay leaves and the parsley stalks. Add the cod and salmon and poach over a low heat for around 20 minutes until cooked through.
Meanwhile, bake the potatoes in the oven.
Remove the fish from the milk and break up, scattering over the bottom of your serving dish, along with the uncooked prawns.
Melt 50g butter in another saucepan, once melted add the flour and mix to create a roux. Add the milk a ladleful at a time, mixing all the time. Add the chopped parsley and a good handful of the cheese, leaving the rest to scatter over the finished dish. Season to taste.
Pour the sauce over the fish and prawns.
Add a layer of uncooked spinach on top of the fish, prawns and sauce mixture.
Remove the potatoes from the oven. Scoop out the potato from the skins and mix with a knob of butter and a splash of milk. Layer the potato on top of the spinach and fork the top to give the pie some texture.
Sprinkle the remaining grated cheese over the top.
Bake at 200c for around 30 minutes until the cheese is golden and the pie is bubbling.
Microwave the potatoes for a few minutes before putting them in the oven; this will reduce baking time.
Use a sieve when ladling the milk into the sauce.
You could split the fish and sauce up into individual serving dishes rather than one large dish if you wish.
Please note: this recipe has been created and uploaded by a member of the Tesco Real Food online community. Tesco does not test customer recipes so cannot accept responsibility for the content, validate the results or answer queries regarding them.