Preheat the oven to 180C/160C Fan/Gas 4. To make the topping, cook the potatoes in lightly salted boiling water for 15–20 minutes, or until cooked. Drain well and mash.
To make the filling, put the eggs in a saucepan of water and bring to the boil. Lower the temperature and boil for 8 minutes. Remove from the pan with a slotted spoon and leave to cool. Peel and cut in half.
Meanwhile, put the smoked haddock, prawns, crayfish, sardines, monkfish, milk, onion and bay in a sauté pan and bring to a simmer. Poach the fish for a few minutes. Lift it out using a slotted spoon and break the haddock and monkfish into large chunks, then set aside. Strain the milk into a jug and reserve.
Add enough of the poaching milk to the potatoes to make a thick mash. Mix in the Gruyère. Season and set aside.
To make the sauce, heat a medium sauté pan, add the butter and heat until melted. Add the shallots and cook until softened. Pour in the vermouth and reduce by half. Tip in the flour, stir well and cook for a minute. Whisk in most of the reserved poaching milk and simmer for a few minutes. Remove from the heat and stir in the smoked haddock, monkfish and herbs.
Put the prawns and half the crayfish in a 23cm/9in ovenproof pie dish. Top with the eggs, then the sauce and white fish. Finally spoon over the mash and stick the remaining crayfish and the sardines in the top, heads and tails showing.
Bake for 20–30 minutes, or until golden and bubbling. Serve.