Preheat the oven to 180 degrees C. Lightly grease a medium baking dish.
In a large deep frying pan pour in the milk to a depth of about 2cm and add a 20g of the butter. Turn the heat to high and add the fish fillets. Bring the milk to the boil then reduce the heat and simmer very lightly until the fish flakes easily with a fork. Remove the fish from the pan using a slotted spoon, discard the skin and flake the fish. Reserve the milk in the pan.
Into the prepared lasagne dish, pour in a little of the milk from the frying pan then add a single layer of lasagne sheets. Add half the flaked fish, half the prawns (if using) and three of the hard boiled eggs.
Over a medium heat, warm the milk the fish was cooked in, adding a little more milk if necessary. Combine the cornflour with the cold water, stir to dissolve then pour into the milk and stir to thicken. Add the remaining butter, the mustard and the wine if using. Stir until a creamy sauce forms, using up any remaining milk if necessary.
Add about a third of the white sauce to the lasagne dish, and layer again with flaked fish, the other three eggs and prawns if using, ending with a final layer of lasagne and the last of the white sauce. Sprinkle the top with grated cheese.
Bake in the preheated oven for about 1 hour or until the lasagne is piping hot and the cheese is golden and melted. Remove from the oven and serve.
A few sun dried tomatoes could make a good addition. This dish does not have to be expensive. Supermarkets sell frozen blocks of white fish that are very reasonably priced, but it can also be as posh as you want with salmon and wine in the sauce.