Place the white fish, smoked fish, prawns or shrimp, halved onion, bay leaf, tarragon and peppercorns in a lidded casserole and pour over the milk. Bring to the boil and then simmer for 15-20 minutes until the seafood is cooked. Remove the fish and prawns and strain the milk, reserving the liquid. Peel the prawns and flake the fish, discarding the shells and any bones. Set aside.
Heat the butter in a large saucepan over medium heat and fry the onion and leeks until soft and translucent. Stir through the crushed garlic and plain flour to create a thickened paste. Cook for a minute then pour in the infused milk, whisking the mixture until smooth. Add the spinach and peas and cook for a minute. Slowly add in the cream, stirring to combine and reducing if necessary. The sauce needs to evenly coat the back of a spoon. When thickened to the right consistency, remove the sauce from the heat.
Stir through the fish and prawn meat then add in the chopped dill and chives. Season the mixture to taste and transfer into a large ovenproof dish.
Roll out the puff pastry on a lightly floured surface to fit the pie dish. Gently cover the pie mixture with the pastry, pressing the edges down to seal in the filling. If desired, decorate the top of the pie with any leftover pastry. Brush the pastry all over with beaten egg and bake the pie in a preheated 220°C oven until golden and flaky.
