1 Cook the potatoes in a large pan of boiling salted water until tender. Drain well and mash over a low heat. Beat in 50g of the butter and season well, adding a drop of milk if the mash is too stiff.
2 Poach the smoked haddock by putting it in a pan with the milk and bay leaves and bringing it to a gentle simmer. Remove from the pan and flake the flesh into chunks. Discard bones and save the milk for later.
3 Steam the diced salmon and cod for 4-5 minutes, until just cooked through. Cook the green beans in a pan of boiling salted water for 4 minutes, then drain and refresh in cold water to preserve the beans' colour.
4 Melt the remaining 60g of butter in a pan and stir in the flour to form a roux. Cook for 1-2 minutes over a gentle heat. Strain the warm milk into a jug and gradually add it to the roux, stirring constantly, until you have a smooth sauce. Then simmer for a few minutes, season to taste, and stir in the chopped parsley.
5 Pour a little of the sauce into 4 individual pie dishes (or one large dish). Divide the fish, beans and prawns equally, and cover with the sauce and allow to cool until it sets.
6 Finally, top with the mashed potato, sprinkle with Parmesan cheese and bake in an oven heated to 180C (Gas Mark 4) for roughly 20 minutes, until the topping is golden brown and the pie is heated through.