Pierce the potatoes all over with a fork and lay on a baking sheet. Bake in the oven until tender. This will take about 35-45 minutes depending on the size of your potatoes.
Whilst the potatoes are cooking, prepare the filling. Add the fish to a pan that will fit the fillets in a single layer and scatter over the onion. Pour over the milk, dot the surface with one third of the butter. Bring up to a steady simmer, cover with a lid or tight fitting piece of foil and poach until the fish is cooked all the way through. Using a slotted spoon, remove the cooked fish to a plate and allow to cool a little. When it is cool enough to handle, flake with a fork, discarding the skin and any little bones you find lurking. Add the flaked fish to a baking dish and evenly scatter over the prawns.
In a clean saucepan melt the remaining two thirds of the butter, then stir through the flour to form a roux. Allow to cook over a gentle heat for a couple of minutes before pouring over the poaching milk. Whisk constantly over a medium heat for about 5 minutes until the sauce is thick and smooth. Season to taste with salt and freshly ground black pepper. Stir through the chopped parsley and pour the sauce over the fish, spreading it evenly.
Once the potatoes are cooked, cut them in half and scoop the flesh into a bowl. Add the butter and mash with a potato masher until smooth, adding a splash of milk. Stir in half of the cheese.
Using a fork spread the potato over the fish filling and fluff up the top. Sprinkle over the leftover cheese and bake in the oven for around 30 minutes, until the pie is bubbling and the top golden and crisp.
Serve with green vegetables, such as broccoli or green beans, or a fresh green salad.