Preheat the oven to 180°C.
Prick potatoes with a fork, rub with oil, and bake on rock salt for 60 to 90 minutes.
Poach fish with onion, peppercorns, and bay leaves in simmering water for 4 minutes.
Reserve 750ml of strained fish poaching liquid and set the fish aside.
Melt 100g butter in a pan, whisk in flour, and cook for 2 minutes.
Whisk 400ml of milk into the flour mixture until smooth and thick.
Stir in 750ml of fish stock and cook until the sauce coats a spoon.
Add lemon zest, juice, parsley, prawns, and flaked fish to the sauce and transfer to a dish.
Increase oven temperature to 220°C.
Mash potato flesh with 75g butter and remaining milk until smooth.
Pipe or spread mash over the filling, sprinkle with cheese, and bake for 10 minutes.
Boil peas for 3 minutes, drain, stir in remaining butter, and serve with the pie.