For the fish pie, preheat the oven to 180C/160C Fan/Gas 4.
Prick the potatoes all over with a fork, then rub with a little oil. Sprinkle a roasting tray with the rock salt and place the potatoes on top. Bake for 1-1½ hours, or until tender.
Clean the mussels in running water. Discard any open mussels that do not close when given a sharp tap. Put a large saucepan over a high heat. Add the mussels and white wine, cover with a lid and cook for 2-3 minutes, or until the mussels have opened. Strain and discard any mussels that have not opened (keep the cooking liquor). Pick the mussels from their shells.
Meanwhile, half-fill a large frying pan with water and bring to a simmer. Add the onion, black peppercorns, bay leaves and all the fish. Poach for 3-4 minutes, then 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 sauce. Continue to add 350ml/12fl oz of the remaining milk little by little, whisking after each addition, until the sauce is smooth and thick. Add 500ml/18fl oz of the fish stock and the cooking liquor from the mussels, 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, juice and parsley, then season with salt and pepper. Remove from the heat. Stir the prawns and lobster meat into the sauce, then flake in the poached fish, keeping it in fairly 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.
Slice the potatoes in half, scoop out the flesh and pass it through a potato ricer into a pan set over a medium heat. Add 75g/2½oz of the butter and the remaining 75ml/2½fl oz milk to the potatoes and beat over the heat until the mash is smooth, 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. (Alternatively, just spread mashed potato over the filling.) Sprinkle over the grated cheese.
Place the ovenproof dish onto a baking sheet and bake for 5-10 minutes, or until the cheese and potatoes are golden-brown and the pie is heated through.
Meanwhile, 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 butter and season with salt and pepper. Serve the fish pie with the peas alongside.