Preheat the oven to 180C/160C Fan/Gas 4.
Prick the potatoes all over with a fork, then rub each one with a little of the oil. Sprinkle a roasting tray with the rock salt and place the potatoes on top. Bake for 1-1½ hours, or until tender.
Meanwhile, half-fill a large frying pan with water and bring to a simmer. Add the onion, black peppercorns, bay leaves and all of the fish, poach for 3-4 minutes. Lift the fish from the pan using a slotted spoon, and set aside. Strain the fish stock through a sieve suspended over a large bowl. Reserve the strained stock and discard the onion, bay leaves and peppercorns.
Melt 100g/3½oz of the butter in a saucepan over a medium heat. Whisk in the flour to form a smooth paste, then cook for a further 1-2 minutes.
Whisk in 50ml/2fl oz of the milk until it is fully incorporated into the roux. Continue to add 350ml/12fl oz of the remaining milk to the roux little by little, whisking after each addition, until the sauce is smooth and thick.
Pour 750ml/25fl oz of the fish stock to the roux sauce, stir well and cook for 4-5 minutes, or until the sauce is smooth and just thick enough to coat the back of a spoon. Any remaining fish stock can be chilled and frozen for use in another dish.
Stir in the lemon zest and juice, and the parsley, then season with salt and pepper. Remove from the heat. Stir the prawns into the sauce, then flake in the poached fish, keeping it in reasonably large pieces. Transfer the pie filling to a large ovenproof dish.
When the potatoes are cooked, remove them from the oven and increase the oven temperature to 220C/200C Fan/Gas 7.
When the potatoes are just cool enough to handle, slice them in half, scoop out the flesh and pass it through a potato ricer into a pan set over a medium heat. Add 75g/2¾oz of the butter and the remaining of milk to the potatoes and beat over the heat until the mash is smooth and thick and not too soft.
Spoon the mash into a piping bag with a large star-shaped nozzle and pipe the mashed potato all over the fish pie filling. Sprinkle over the grated cheese. Alternatively, just spread the mashed potato onto the filling using a palette knife.
Place the ovenproof dish onto a baking sheet and cook in the oven for 5-10 minutes, or until the cheese and potatoes are golden-brown and the pie is heated through.
While the fish pie heats through, bring a pan of salted water to the boil, add the peas and cook for 2-3 minutes, until tender. Drain well and return to the pan, then stir in the remaining butter and season with salt and pepper. Serve the fish pie with the peas alongside.