For Fish Fumet: Place water, fish bones, celery tops, leek, onion, bay leaf, peppercorns and thyme in saucepan over moderate heat. Bring to a boil, then simmer for 20 minutes, skimming frequently. Strain.
Drain potatoes and boil in fresh, salted water for 20 minutes. When soft, drain off water and return to same pan on low heat to evaporate excess water. Remove from heat and mash thoroughly. Add a little butter, white pepper and some warm milk. Season with dill, parsley and paprika. Cover to keep warm and set aside.
For sauce: Melt remaining butter in saucepan and stir in flour. Cook, stirring constantly, for 3 minutes over low heat. Add remaining milk and cider, stir thoroughly, increase to a boil and cook for 3 minutes, then reduce heat.
Meanwhile, in frying pan, add scallops and 3/4 cup fish fumet. Poach for 6 minutes until scallops are done. Set scallops aside. Add poaching liquid to sauce and stir. Reduce heat and simmer gently.
In another frying pan, add some clarified butter. Add mushrooms and lemon juice and sprinkle with paprika. Cook gently. Season with white pepper. Stir mushrooms into sauce. Pour into the pan with the scallops.
Preheat broiler. To serve, cover the sides of a heat-proof serving platter or casserole with some of the mashed potatoes and pour in the scallop and mushroom mixture. Put remaining mashed potatoes in a pastry bag and pipe potatoes around the rim. Sprinkle with cheese and broil until brown, about 4 minutes.
This recipe was provided by a chef, restaurant or culinary professional. It has not been tested for home use.