Season all the seafood with salt, pepper and dill and mix well. Set aside to drain in a colander for 1 hr - to drain the excess water.
Steam the potatoes until tender. Remove and mash while still hot. Once the potato is mashed, whip in egg yolks and the double cream, and season if required.
To make the velouté, melt the butter in a pan, then add the flour. Mix and set aside. Bring the fish stock to boil, then add cream and Dijon mustard, if using. Reduce to half, and then whisk in the butter and flour mixture. Cook for at least 15 mins, stirring continuously, until the flour taste has gone. Add a little more water if it is too thick.
To make the topping, melt the butter in a pan and add the bread crumbs to it. Sauté until it becomes golden in colour.
In a baking dish, place the seasoned fish and seafood with the velouté – adding it a little at a time, so you don’t have a sloppy mix. Top with the mash and breadcrumb mix. Add a layer of parmesan cheese and bake for 20 to 25 mins at 190C until you have a crisp, golden coloured topping.