Preheat the oven to 180°C.
Prick potatoes, rub with oil, place on rock salt, and bake for 60 to 90 minutes until tender.
Steam mussels in white wine for 3 minutes, then strain and reserve the liquor.
Poach all fish with onion, peppercorns, and bay leaves in water for 4 minutes, then strain and reserve the stock.
Melt 100g of butter and whisk in flour to cook for 2 minutes.
Slowly whisk in 400ml of milk, 500ml of fish stock, and the mussel liquor until smooth and thick.
Stir in lemon, parsley, seasonings, prawns, lobster, and poached fish flakes, then transfer to a dish.
Increase oven temperature to 220°C.
Rice the potato flesh and beat with 75g of butter and 75ml of milk until smooth.
Pipe or spread the mash over the fish filling and sprinkle with grated cheese.
Bake for 5 to 10 minutes until the top is golden and the pie is hot.
Boil peas for 3 minutes, drain, stir in 50g of butter, and serve with the pie.