Melt the butter in a large frying pan. whisk in the flour and gently cook for 2 minutes. Then gradually add the strained fish stock little by little, whisking all the time. When you have a smooth sauce, turn the heat to its lowest setting and let the sauce gently cook for 5 minutes.
Then, whisk in the crème fraîche, followed by the cornichons, parsley and dill, wasabi paste and frozen peas
Give it all a good seasoning and remove it from the heat.
Add white fish, salmon and prawns to the sauce
Layer mash potato fish mixture and more mash before finally scattering the cheese over the surface and bake on a high shelf of the oven for 35-40 minutes.